3 Estimation

Estimation is a useful tool in mathematical problem-solving. Estimating gives you an approximate or “ballpark” answer. It is important to be able to make estimates in your head before you use a calculator, to enable you to double- check your answer in case you press the wrong key on the calculator. People often estimate differently; there is no one correct way to estimate. It takes practice to get good at choosing numbers that make estimating easier.

Here are some estimating strategies:

1. Round to numbers that are easy for you to compute.

For example, about how much is 29 × 14? Some possible estimates are:

• 25 × 16 = 400
• 30 × 10 = 300
• 30 × 15 = 450

2. Use numbers that make sense to you

When you round, sometimes the result will be an underestimate (less than the exact answer), and sometimes it will be an overestimate (more than the exact answer).

Estimate these problems:

[latex]\begin{array}{lll} 31\times4&30\times4=120&\text{a little more than }120 \\ 48\times5&50\times5=250&\text{a little less than }250\\ 17\times21.2&20\times20=400&\text{about }400\end{array}[/latex]

3. Choose numbers that are compatible and easy to work with for the problem.

For example, how much is 2775 divided by 6? Some possible estimates are:

3000 ÷ 6 = 500