2
Regrouping practice
Discover Mathematics P2: Term 2, Lessons 111–120
GoalPractise adding and subtracting two- and three-digit numbers with regrouping.
New words
regroup · trade between places: 10 ones for 1 ten, or 1 ten for 10 onessum · the answer to an additiondifference · the answer to a subtraction
Explanation
When we add and the ones make 10 or more, we trade 10 ones for 1 ten and write the extra ten above the tens. When we subtract and there are not enough ones, we trade 1 ten for 10 ones. The same works for tens and hundreds. For 48 + 37: ones 8 + 7 = 15, write 5 and carry 1 ten. Tens 4 + 3 + 1 = 8. So 48 + 37 = 85. Always check an addition with a subtraction: 85 − 37 = 48.
Worked examples
268 + 157. Ones: 8 + 7 = 15, write 5 and carry 1. Tens: 6 + 5 + 1 = 12, write 2 and carry 1. Hundreds: 2 + 1 + 1 = 4. So 268 + 157 = 425.
91 − 56. Ones: 1 is too small, so trade a ten: 11 − 6 = 5. Tens: 8 − 5 = 3. So 91 − 56 = 35.
603 − 258. There are no tens to trade, so first trade 1 hundred for 10 tens, then 1 ten for 10 ones. Ones: 13 − 8 = 5. Tens: 9 − 5 = 4. Hundreds: 5 − 2 = 3. So 603 − 258 = 345.
Class activity
Regrouping race. Two players each get the same ten problems on cards: five additions and five subtractions with regrouping. Both solve them on paper with a timer. The winner is the one with more correct answers, not the one who is fastest. Check every answer with the opposite operation.
Practice
1
Find 76 + 18.
76 + 18 = 94.
2
Find 83 − 47.
83 − 47 = 36.
3
Find 345 + 278.
345 + 278 = 623.
4
Find 500 − 234.
500 − 234 = 266.
5
Seif says 47 + 25 = 62. Is he right? Check.
No. 47 + 25 = 72, because 7 + 5 = 12 makes a new ten. Check: 72 − 25 = 47.