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- Keenya brings a bag of 24 cookies to school to share with the class. Five of her classmates also bring 24 cookies each. How might Keenya estimate the total number of cookies they will have for the class?
- Keenya thinks, “We each bring about 25 cookies. Since 25 6 is 150, we will have about 150 cookies.” What is Keenya's convenient number? How did she pick it?
- Jackie's Girl Scout troop is planning a hike. The troop needs to know how heavy the supplies will be. Jackie weighs a bottle of water and finds that it weighs 4.5 kg. About how much will 3 bottles of water weigh?
We find estimates when we need to have a good idea about how big or small a number is, but we do not need to know exactly. Sometimes it is impossible to know an exact answer. We use estimates in many different situations.
For example, you can estimate the things below:
We can estimate answers to problems that involve multiplication by finding a product that is reasonably close to the exact answer. It may be bigger or smaller than the actual answer, depending on the problem.
Since estimating is often done mentally, it is helpful to choose convenient numbers that are easy to multiply in our heads. Convenient numbers are close to the actual numbers in a problem, but they are really estimates, too.
Use convenient numbers to solve the problems below. Decide if your estimate is an overestimate or an underestimate. Consider and tell which is better for the problem situation: an overestimate, an underestimate, or it doesn't matter. Fill in the circle by your answer.
- Mrs. Borko buys 4 jackets for her children. Each jacket costs $47. About how much will all 4 jackets cost?
- There are about 27 students in each fourth-grade class at Hill Street Elementary School. If there are 6 fourth-grade classes, about how many fourth-graders are there at the school?
 $100 |
$20 |
$160 |
$2000 |
$200 |
| 120 |
1200 |
150 |
180 |
300 |