Equal Additions

For a problem like 445 – 398, the fact that 398 is very close to a tidy number, namely, 400,suggests that a useful way of solving it is by equal additions, in this case, of 2. The problem then becomes 447 – 400, whose answer is obviously 47.

Using Materials

Problem: “Debbie has $445 in her bank account, and her younger sister Christine has $398. How much more money does Debbie have?”

Make piles of $445 and $398. “Now suppose that Grandma gives Christine $2 to give her a ‘tidy’ amount of money. To be fair, Grandma gives Debbie $2 also.” Discuss why 445 – 398 has the same answer as 447 – 400 and then record 445 – 398 = 47 on the board or modelling book.

Examples: Word stories and recording for: 367 – 299,  546 – 497,  662 – 596,  761 – 596,  334 – 95, 567 – 296 ...

Using Number Properties

Examples: Word stories and recording for: 900 – 298,  701 – 399,  760 – 96, 905 – 96,  507 – 296,  865 – 590,  1 000 – 396 ...