2
Review: regrouping and fractions
Discover Mathematics P2: Term 2, Lessons 61–120
GoalReview adding and subtracting with regrouping, estimating by rounding, and fractions of shapes and sets.
New words
regroup · trade 10 ones for 1 ten, or 1 ten for 10 onesestimate · a close guess found by roundingfraction · part of a whole or of a group, such as 1/4
Explanation
When we add three-digit numbers we start with the ones and carry when a place makes 10 or more. When we subtract we trade when a place is too small. Estimate by rounding to the nearest ten to see if an answer is close. A fraction needs equal parts: 1/2, 1/3 and 1/4 are one of 2, 3 or 4 equal parts. To find a fraction of a group, split the group into equal sets: 1/4 of 8 is 2. For 276 + 148: 6 + 8 = 14, 7 + 4 + 1 = 12, 2 + 1 + 1 = 4, so the sum is 424.
Worked examples
352 − 167. Ones: trade a ten, 12 − 7 = 5. Tens: trade a hundred, 14 − 6 = 8. Hundreds: 2 − 1 = 1. So 352 − 167 = 185.
| Number | About |
|---|---|
| 78 | 80 |
| 43 | 40 |
| 80 + 40 | 120 |
1/3 of 9 apples. Split 9 into 3 equal groups of 3. One group is a third, so 1/3 of 9 = 3 apples.
Class activity
Mixed cards. Write six cards: two additions, two subtractions with regrouping, one estimate and one fraction of a set. Mix the cards. Take one at a time and solve it on paper. Check each one with the opposite operation, or by counting beans for the fractions.
Practice
1
Find 487 + 235.
487 + 235 = 722.
2
Find 604 − 275.
604 − 275 = 329.
3
Estimate 91 − 38 by rounding to the nearest ten, then find the exact answer.
90 − 40 = 50. Exactly 91 − 38 = 53.
4
What fraction of the circle is shaded?
2/3.
5
Salma has 12 stickers and gives away 1/4 of them. How many does she give away?
1/4 of 12 = 3 stickers.