Pennies in a Mile
Date: 04/23/99 at 21:01:01 From: Ann Humphreys Subject: How many pennies are there in a mile? Dear Dr. Math, I teach 4th grade. Our school is doing fund-raisers to help beautify our playground. One of the committee members asked if I could find out how many pennies there are in a mile. I guess they are going to have a contest of some sort. I will continue looking for the answer; however, I saw this Web site and thought it wouldn't hurt to ask. Thank you, Ann
Date: 04/23/99 at 23:01:33 From: Doctor Peterson Subject: Re: How many pennies are there in a mile? Hi, Ann. I can tell you the math, but you'll have to do the measuring - or better yet, have your students do it. Just measure the width (or height, if you'll be stacking the pennies a mile high) of a penny, then divide 5280 feet by whatever you find. This is a great activity for a class, and in fact a good way to start the project. First take one penny, and work out an answer. Then get 100 pennies, and measure them; do the same calculation to see how many pennies it will take to make a mile. There will probably be a difference, because you can measure 100 pennies more accurately than a single penny. Or maybe you have a micrometer that will measure one penny precisely. Which is better can be a good discussion starter. And don't forget to try it in metric, too. Just to illustrate, using a very rough estimate of a penny's width, let's say a penny is about 3/4 inch wide. The number of pennies in a mile will be 5280 ft 12 in 1 penny 1 mile * ------- * ----- * ------- = 5280 * 12 * 4/3 pennies 1 mi 1 ft 3/4 in This gives about 84,480 pennies. (This method of doing calculations with units is very helpful, and would be worth teaching.) If we measure 100 pennies as 6 ft 1 in, we will get 5280 ft 100 pennies 1 mile * ------- * ----------- = 5280 * 100 * 12 / 73 pennies 1 mi 6 1/12 ft This gives us 86794.5205 pennies in a mile. Which is more accurate? Ask the students, and see what ideas they come up with. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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