GoalAdd and subtract three-digit numbers with a strategy of my choice and check the answer.
New words
strategy · a plan for solving a problemregroup · to trade 10 of one place for 1 of the next placecount up · to find a difference by adding up from the smaller number
Explanation
There is more than one good way to add and subtract. Break apart: to add 346 + 275, add the hundreds, tens and ones separately: 300 + 200 = 500, 40 + 70 = 110, 6 + 5 = 11, and 500 + 110 + 11 = 621. Round and adjust: 598 is close to 600, so 598 + 237 = 600 + 237 − 2 = 835. Count up: to find 703 − 258, start at 258 and jump to 300 (42), then to 700 (400), then to 703 (3). The jumps add up to 42 + 400 + 3 = 445. Always check. For a sum, subtract one of the parts; for a difference, add back: 445 + 258 = 703, so the answer is right.
Strategy
Example
Break apart
346 + 275: 500 + 110 + 11
Round and adjust
598 + 237 = 600 + 237 − 2
Count up
703 − 258: 42 + 400 + 3
Worked examples
Place
Work
Answer
Hundreds
300 + 200
500
Tens
40 + 70
110
Ones
6 + 5
11
Total
500 + 110 + 11
621
Add 346 + 275 by breaking apart. Add each place: hundreds 500, tens 110, ones 11. Then 500 + 110 + 11 = 621.
Add 598 + 237 by rounding. 598 is 2 less than 600. Add 600 + 237 = 837, then take away the extra 2: 837 − 2 = 835.
Subtract 703 − 258 by counting up. Jump from 258 to 300: 42. Jump to 700: 400. Jump to 703: 3. Add the jumps: 42 + 400 + 3 = 445. Check: 445 + 258 = 703.
Class activity
Pick a strategy. Write six problems: three additions and three subtractions with three-digit numbers, for example 465 + 238 and 802 − 376. Solve each one with a different strategy than the one you used last time. Check each answer with the opposite operation. Which strategy was your favourite?
