Editorial desk · Practical notes

Estimate before multiplying

Consider 19 × 6. Before calculating, use 20 × 6 = 120 as a nearby estimate. Since 19 is one less than 20, subtract one group of 6: the exact answer is 114.

Write the estimate and exact answer together. Explain why the exact answer should be smaller than the estimate in this example. That comparison provides a quick check on the direction of your calculation.

Try 29 × 4 using 30 × 4 as the starting point. Then verify by another method. Estimation is useful when it stays connected to the size and meaning of the numbers.