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Estimation in 3rd Grade

Date: 12/16/96 at 09:50:18
From: Karen Stetchock
Subject: Estimation - third grade

Dr. Math,

How would you suggest to teach estimation to a third grade classroom?    

Thanks for your help,   - Karen

Date: 12/16/96 at 19:51:53
From: Doctor Tom
Subject: Re: Estimation - third grade

Hi Karen,

I don't know how much help I'll be, since I've never been the
instructor in a third grade class, but I do have a lot of
experience with 7th graders, and I've tried (with less success
than I'd like) to talk about estimation.

I think estimation is a critical ability - I try to get students to 
try to estimate answers before they do any real calculation.

I'd start with questions like this:  Suppose you go to the grocery 
store and bring 5 different boxes of cookies to the cash register.  
The cashier rings them up, and tells you, "That'll be $452.18."  How 
do you know he's wrong?  What sorts of numbers are "reasonable"?  How 
do you know that?

Talk about how to simplify:  What's 1010 times 980, approximately?
Well, it's roughly 1000 times 1000, or a million.  Talk about how
to find "round" numbers close to the actual numbers.

I made a list of questions for estimation.  They're aimed at smart 
7th graders, but maybe you can use some of them, or can convert them 
to more suitable forms for your kids.  I'll append that list and some 
comments about it at the end.  (I guess from the questions it's 
obvious that I live near San Francisco.)

Good luck!

How long to drive from NYC to SF without stopping?

How much does an elephant weigh?  A mouse?

How many tiles on the floor of this classroom?

How many parts in a bicycle?

How many turkeys to feed all the kids at this school for Thanksgiving 

In those crime movies, you see briefcases full of $100 bills.  How 
much is such a briefcase worth?

How long could you live on $1000?

How many miles of roads in San Francisco?

How much money (in coins) could you carry?

How many feet of yarn in a sweater?

How much water in a bathtub?

How many leaves on an oak tree?

How many kernels on an ear of corn?

How much gasoline in a big tanker-trailer?  How far could you drive 
using it?

Catch 100 fish in a lake, tag them, and toss them back.  Tomorrow you 
catch 100 more, and 10 are tagged.  About how many fish are there in 
the lake?

The hardest thing about this exercise was trying to get across the 
idea that I didn't expect an exact answer - just a good answer.  Many 
of the kids seemed to think that if the answer wasn't exact, then any 
wild guess was as good as any other.

I still haven't made this work as well as I'd like, but I think the 
idea is critically important.  Whenever I start a mathematical 
calculation, I do a rough calculation in my head so I know if my
calculated answer is in the right ballpark.

-Doctor Tom,  The Math Forum
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Associated Topics:
Elementary Multiplication
Middle School About Math

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