Solve addition and subtraction problems by compensating with tidy numbers (including equal additions).
Number Framework Stage 6.
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: "Christine has $367 in her bank account, and her younger sister, Julie, has $299. How much more money does Christine have?"
Have the students make piles of $367 and $299.
Ask: “Now, suppose that their Nana gives Julie $1 to give her a tidy amount of money.
To be fair, Nana has to give Christine $1 as well.”
Discuss why 367 – 299 has the same answer as 368 – 300.
Record 367 – 299 = 68 on the board or in the modelling book.
Examples: Word stories and recording for: 367 – 299, 546 – 497, 662 – 596, 761 – 596, 334 – 95, 567 – 296 ...
Using imaging
Problem: "Brian has $483 in his bank account, and his younger brother, Tom, has $397. How much more money does Brian have?"
Students image piles of $483 and $397.
Ask: “How much money would Nana give to both boys to make this an easy subtraction problem?”
Discuss why 483 – 397 has the same answer as 486 – 400.
Record 483 – 397 = 86 on the board or in the modelling book.
Examples: Word problems and recording for: 257 – 199, 676 – 498, 562 – 496, 763 – 396, 434 – 195, 762 – 598 ...
Using Number Properties
Examples: Word stories and recording for: 900 – 298, 701 – 399, 760 – 96, 905 – 96, 507 – 296, 865 – 590, 1 000 – 396 ...