Skill: Estimate before calculating

Before calculating exactly, make a quick prediction about the answer’s size. This helps you notice if your final answer is unreasonable.

Example: Find $48.7 \times 3.9$.

1. Round to compatible numbers.
   Round $48.7$ to $50$ and $3.9$ to $4$.

2. Estimate.

$$50 \times 4 = 200$$

So, we expect the exact answer to be close to $200$.

3. Calculate exactly.

$$48.7 \times 3.9 = 189.93$$

4. Compare the answers.
   The exact answer, $189.93$, is close to the estimate, $200$. Therefore, the answer is reasonable.

If you accidentally placed the decimal point and got $1{,}899.3$ or $18.993$, the estimate would show that something went wrong because those answers are not close to $200$.