Checking Division by Estimation

Using Number Properties

Problem: “The manager at the bookstore buys 69 books for $1,327. Estimate the price of one book.” Discuss why 69 is rounded to 70. (Answer: 70 has only 1 significant figure so tables will be able to be used in the next step.) Write 1 327 ÷ 70 on the board and discuss what 1 327 could be rounded to. (Possible answer: Rounding to say 1 300 or even 1 000 produces difficult divisions to do mentally. But rounding 1 327 to 1 400 produces the division 1 400 ÷ 7, which can be reduced to 14 ÷ 7 = 2 then adjusted to 1 400 ÷ 70 = 20.)

Examples: Worksheet (Material Master 8–4).

Understanding Number Properties:

Make up a four-digit decimal number divided by a two-digit decimal number problem of your own and explain how you would estimate the answer.