Division, Not Long Division

Division is the most powerful arithmetic operation. It makes comparisons. When the numerator and denominator have the same units, the comparison makes a dimensionless number, the only kind that the universe cares about. Long division, however, is something else entirely. In my post “Dump algebra,” many commentators objected to my loathing of long division. But long division is not division! Long division is just one way to do the computation, and is far from the most useful way.

I’ll illustrate with an actual example of division. For my environmental-protection lawsuit, now in the Massachusetts Supreme Court, I needed to divide 142,500 by 4655. Here is the long-division calculation, my first use of the method in 30 years:

The calculation took me a few minutes with paper and pencil, some of the time to reconstruct the algorithm details and to get the bookkeeping straight — even though I already knew the answer quite accurately.

I knew the answer because I had already applied a more enjoyable method: skillful lying. I turned the numerator 142,500 into the nearby and convenient 150,000; and turned the denominator 4655 into the nearby and convenient 5000. Dividing 150,000 by 5000 gives 30. It’s likely to be an accurate estimate, because the two errors (increasing the numerator and increasing the denominator) partly compensate.

The next correction is not too hard, and comes from estimating the two errors. The actual numerator of 142,500 is 7500 less than 150,000, a decrease of 5 percent. To fix this error, decrease the estimate by 5 percent. The actual denominator of 4655 is 345 less than 5000, a decrease of, let’s say, 7 percent (7 percent of 5000 is 350). To fix this error, increase the estimate by approximately 7 percent. The two fixes together require increasing the estimate by 2 percent (7 percent minus 5 percent). So, 30 becomes 30.60—which is very close to the actual quotient of 30.6122…

This whole calculation took about 10 seconds in my head. There’s no need for long division, and I hope that I live another 30 years without using it again.

For students, learning long division mostly means learning like a parrot. A classic example is from the National Assessment of Educational Progress (NAEP) results reported in 1983 (Carpenter, T., et. al., “Results of the Third NAEP Mathematics Assessment: Secondary School,” The Mathematics Teacher, 76:652-659). Thirteen-year-olds across the country were asked:

An army bus holds 36 soldiers. If 1128 soldiers are being bused to their training site, how many buses are needed?

70 percent of the students did the long division correctly (the result of 1128/36 is 31 and 1/3). From doing the division correctly, the most popular answer, chosen by 29 percent, was the meaningless 31 R 12 (31 with a remainder of 12) buses. Another 23 percent chose 31 buses, leaving 12 soldiers stranded. Only 18 percent chose the correct answer of 32 buses. Even then it’s not clear how many of the 18 percent were sure of their answer or were just guessing between 31 and 32.

Here is a flow diagram illustrating the answer distribution:

It’s easy to learn long division yet understand little.

 


Eric M. Jones.

A zillion years ago (estimate) logarithms were no longer calculated by hand, they were looked up in a table. Same with trig functions...remember the Taylor and Maclaurin series?

Extracting roots, long division and multiplication, percentages and even addition and subtraction (above a couple digits), should be relegated to the dustbin of history too. Just grab a calculator. If you really need to, look up the manual method. This is useful if you need to write a software program where numbers are manipulated in some way.

Same goes for those whacky "fractions". Take it from your Uncle Eric kids...THERE ARE NO FRACTIONS, just one number divided by another. Manipulating them is a good intro into similar manipulations for pre-Algebra, but that's all.

Not something that many people need to know. There are far better things to learn. and BTW I am thoroughly FOR Algebra and Trig, and if you're going into a technical field, Calculus.

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Dave

Incidentally, a zillion years ago people understood logarithms. I used to tutor a lot of kids in algebra 1, 2, and calculus, and I haven't ever bumped into a kid with a solid understanding of logs.

Long division in itself isn't a difficult skill, or particularly useful, but long division is a great indicator of whether someone has their basic division abilities down pat. The 142,500 / 4655 is definitely nothing that most students will ever get easily, but there is no problem making sure kids can do 1027/3.

YX

That seems to suggest kids need to learn MORE math, not less... that's pathetic.

Long division is second grade math in Russia, China and India.

Matthew

In Soviet Russia, math divides YOU. (Sorry, I couldn't resist.)

BL1Y

"This whole calculation took about 10 seconds in my head."

It took him about 10 seconds to divide 150,000 by 5,000, and then to figure that 7,500 is about 5% of 150,000 and that 345 is about 7% of 5,000, and then figure what 2% of 30 was?

I don't buy it.

Clancy

I think you might get more traction with this one than dumping algebra. Not only is long division tedious and no one ever uses it, but it doesn’t even teach you anything new about math. Do I have a deeper understanding of the process of division because I know long division?
No.
I have to divide large and sometimes odd numbers all the time. I try to have a calculator with me as much as possible, but if I don’t then I have to work it out. My reflex is to do the long division, but I have to stop myself and say “That’ll take an hour and you’ll probably screw it up anyway. Just estimate it.”

Seminymous Coward

It was supposed to teach you that subtracting multiples of the divisor from the dividend is an effective way to break down the problem of dividing things. Furthermore, it was supposed to teach you that you can select those multiples perfectly in a single pass.

It also provides one of the only clear algorithmic solutions many people ever see. That should be illustrating to you the value of a defined, methodical process for gradually attacking seemingly intractable tasks. It is also setting up a common reference point for explaining what an algorithm is to you later.

It's unfortunate that in your case it seems to have damaged your self-esteem to the point of giving up on elementary arithmetical calculations. That's a pity when it could have been an opportunity for your teacher to let you see yourself overcoming what was previously an insurmountable large challenge via learning and a bit of persistence.

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Joe

Hence, the problem with asking people who don't use math if their kids need to learn math (ie. should your kids learn algebra).

Math education in the US is a royal mess. Tinkering around the edges, adding or dropping topics, isn't going to fix things.

Iljitsch van Beijnum

But it's much harder to not learn long division and yet understand much.

Long division is the only way to do division accurately with pen and paper. Using tricks to simplify the calculation enough to perform them in your head is nice, but it's very error prone and it's not a universal solution.

Clancy

Long division is also error prone. There are dozens of calculation steps where you could make mistakes. If you really need to be precise to the sixth decimal place, (e.g. calculating the landing speed of a Mars Probe) then you should use a calculator or a computer.

Thinking about reducing fractions provides much more insight into the process of division and is an important step towards learning algebra.

Mike

What I'm struggling with here is that you have effectively done in your second approach is essentially the long division in your head.

Unless I'm totally missing your point (or I have a modified strategy for long division), the first step of long division is inevitably the "skillful lying" step / estimation. You don't get to put 30 above the line until you have figured out the number of times 4655 can go into 142500 completely. A long divider is going to try something like one of the following possibilities to get the whole number: 4000 into 140000 or 5000 into 145000. That would get you your starting place and add or subtract a 1 until you get to the right "whole" number.

David O. Cushman

The answer of "32" follows from an assumption that is not explicitly stated, that the cost of stranding 12 exceeds the cost of using the 32nd bus. If not, "31" or "31 R 12" is a better answer. (I do doubt that many of the students giving those answers thought of it this way.) A more fundamental question is what is the optimal number of soldiers to transport. To me, when the question moved beyond just being arithmetic, the question's wording became inadequate. It also shows that just learning to do arithmetic is inadequate.

Seminymous Coward

If you want to pick the problem apart, it'd be a lot more entertaining to just say "You only need 1 bus ...for 32 trips." That at least presents a real-world objection to the simplistic logic of this word problem for children. Furthermore, it could even lead to a discussion of the optimal number of buses for the army training camp to have around.

I will indulge you for a moment, though. The externally imposed 1128 soldier condition tells you the optimal number of soldiers to transport. There's no way having a bunch of untrained soldiers sitting at the bus stop for their full enlistments makes any sense.

Bart

I agree and disagree. I think that we have to teach long division to get them to understand the precision - and this is where the stories come in - we use the stories to clarify what is most appropriate and correct for the situation. Once they understand the process then we can help them understand how precise they need to be - or not. Depends whether you are moving troops or working with quantum particles. It also depends where you work. I do a lot of field work. I do have a calculator. I have had that calculator fail on me at the most inopportune moments. So, to pen and paper I went. For some stuff. For the precision.

For other stuff, like in demolition, figuring out how far to cut around the base of a column to make it fall over, we just used Pi = 3 plus a little. But if were weren't going to let this tower fall, rather lift it with a crane, then an approximation tells you if you are in the ballpark of your cranes limits, and you may need to get to the precision to make sure.

Critical thinking stems from the process.

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Corwin Delight

Until last year in calculus, I hadn't used long division in about 10 years either. But my Calc teacher showed us a trick to using long division to simplify complex polynomials:

3X^3- 2X^2+15
--------------------
X^2+5X

set it up like long division,

_____________
X^2+5X |3X^3-2X^2+15

then solve like long division. You're left with a regular non-fraction polynomial, and a smaller, more simplified fraction (the remainder from long division) for the answer.

3X +17
____________
X^2+5X |3X^3-2X^2+15
-(3X^3+15X^2)
_____________
0X^3-17X^2 +15
-(17X^2+85X)
______________
0X^2 -85X +15

Answer:
3X^3- 2X^2+15 85X +15
-------------------- = 3X+17 - -----------
X^2+5X X^2+5X

Note: This may or may not have actually made things easier, as I made these numbers up on the spot, but the important thing here, is that this works not just for cubics, but for any polynomial of any degree.

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Corwin Delight

Bah, the white space that I entered to make the formatting look like long division symbols didn't really work. Hopefully the whole thing is still understandable.

the final answer btw, is 3X+17 - ((85X+15)/(X^2+5X))