How Many Jelly Beans?

In the olden days, when people used to meet and socialize at the General Store, they'd take part in the weekly "Jellybean Guess". There was a giant sized jar of jellybeans on the counter and people had to guess how many jellybeans were in it. They'd write their guess on a slip of paper and put it in the answer box. The winning guess (if there was one) had to be within 10 jelly beans of the correct total, which only the store owner knew.

Most people would "eyeball" the jar and submit their guess immediately. Others who were more scientifically minded would estimate the number of jellybeans with a mathematical approach. They knew the capacity or volume of the giant jar in the store, so they would count how many jelly beans filled a smaller jar -- one with ¼ the volume of the big one. When they multiplied that number by 4, they would have their estimate. When there was a winner, it was usually one of these people, who then got to eat all those jellybeans.

When we can't count each individual object in a large group, we make a logical guess or estimate. Say we know that it takes 10 minutes to walk to the library. If the museum is 3 times as far as the library, we'd estimate or guess it would take us 30 minutes to reach the museum.

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Examples

1) Juan found he could fit 50 jellybeans into a jar one fifth the size of the big one so he guessed
5 × 50 or 250 jellybeans in all. The actual total was 257.

2)

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Now get a pencil, an eraser and a note book, copy the questions,
do the practice exercise(s), then check your work with the solutions.
If you get stuck, review the examples in the lesson, then try again.

Practice Exercises

Choose the best estimate.

1) number of books in a city library		2) length of a pencil
80 books	8 000 books	7 inches	70 inches
3) height of a cow		4) number of students in your class
60 feet	6 feet	250 students	25 students
5) number of tires on a tractor-trailer truck		6) distance from New York to Los Angeles
18 tires	180 tires	250 miles	2 500 miles

7) Challenger Question

8 grown-ups and 12 kids are coming to the family Bar-B-Q. The grown-ups usually eat 2 or 3 burgers each, the kids always eat just 1. Estimate the number of burgers and buns to prepare so that there is enough food for everyone.

Solutions

Choose the best estimate.

1) number of books in a city library		2) length of a pencil
80 books	8 000 books	7 inches	70 inches
3) height of a cow		4) number of students in your class
60 feet	6 feet	250 students	25 students
5) number of tires on a tractor-trailer truck		6) distance from New York to Los Angeles
18 tires	180 tires	250 miles	2 500 miles

7) For 12 kids, we need a dozen (12) burgers and buns. For 8 adults, we need between 16 and 24 burgers and buns. The least we'll need is 16 + 12 or 28. If all the adults want 3 burgers, we'll need
24 + 12 or 36 burgers. A good estimate would be 32 -- half way between 28 and 36.

( Primary MathRoom Index )