Date: 08/23/98 at 22:14:25 From: Anna Subject: Long division lag Dear Dr. Math, I am in fifth grade and am having trouble with third grade arithmetic. My biggest problem is long division. For example, what is 73/4? Please help me.
Date: 08/24/98 at 13:26:18 From: Doctor Peterson Subject: Re: Long division lag Hi, Anna. Lots of people need help with long division. Sometimes it just takes several different people explaining it before you find an explanation that works for you. Here's a nice explanation in our Dr. Math archives that you might like to work through: Learning Long Division http://mathforum.org/library/drmath/view/58816.html One thing that bothers many people is that they are told they have to "guess" as part of the process. For division by a one-digit number like your example, there's really no guessing involved. The guessing comes in for bigger divisors, because you haven't memorized (and shouldn't!) the multiplication tables for, say, 87 times anything. So there are techniques for making a good guess and then fixing it if it turns out to be wrong. But as long as we're dealing with a simpler problem, I won't go into that. Write back when you're ready for it. Rather than repeat what others have written about how to divide, I'd like to tell you a little about why. Some people can do a task better if they know why they do what they do. So let's divide 73 by 4, and think about what we are doing. I have 73 sticks I want to bundle in groups of 4, and I want to know how many bundles I will end up with. (That's division.) Luckily, someone has given them to me already bundled in tens: 7 bundles of ten, and 3 extra sticks. (That's called place value.) 70 3 |||||||||| ||| |||||||||| |||||||||| |||||||||| |||||||||| |||||||||| |||||||||| Rather than unbundle them all, I can start out by working with the 7 bundles and divide them by 4. That gives me one group of 4 bundles. Those 4 bundles of ten can be easily rebundled into 10 bundles of 4, so I've got a big part of the job already done. (I just divided 7 by 4, getting 1, with 3 left over.) 10 4's |||||||||| |||||||||| |||||||||| |||||||||| -------------------------- 30 3 |||||||||| ||| |||||||||| |||||||||| Now I have 3 bundles of 10, and 3 more. At this point I untie the bundles and find that I have 33 sticks. I check my handy multiplication tables and find that 4 times 8 is 32. (I've divided 33 by 4 to get 8 with a remainder of 1.) 10 4's |||||||||| |||||||||| |||||||||| |||||||||| 8 4's |||||||| |||||||| |||||||| |||||||| 1 | Now look at what I have: 10 bundles of 4 from the first step, and 8 more from the second. The answer is 18 bundles of 4, and 1 left over. Do you see how this gave me the answer, one digit at a time, by looking at the 73 one digit at a time? Let's do the actual work: Divide 7 by 4, getting 1 rem 3. I write the 1 on top, then multiply it by 4 and write down 4 below the 7, subtracting that to find the remainder: __1__ 4 ) 73 4 - 3 Now combine the remainder of 3 and the next digit, 3, to make 33. Divide that by 4 and write everything down the same way: __18_ 4 ) 73 4 - ___8_ 33 <---> 4 ) 33 32 32 -- -- 1 1 The answer is 18, with a remainder of 1. Look back and see how this compares to my stick counting. I hope that makes a little sense out of the process, so you can remember better what you have to do. You're just working with one digit of the answer at a time. If you still need more help, you might try writing out how you do a problem, and I could see where you have trouble. Don't give up. You can do it. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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