2
Estimate, then subtract
Discover Mathematics P2: Term 2, Lessons 91–100
GoalEstimate a difference by rounding to the nearest ten, then subtract exactly and compare.
New words
estimate · a careful guess that is close to the answerround · change a number to the nearest tenexact · the real answer, not a guess
Explanation
An estimate helps you check an answer. Round each number to the nearest ten, then subtract. Look at the ones digit. 0 to 4: round down. 5 to 9: round up. For 78 − 32, 78 is about 80 and 32 is about 30. So 80 − 30 = 50, and the answer is about 50. The exact answer is 78 − 32 = 46, which is close to 50.
| Number | About |
|---|---|
| 78 | 80 |
| 32 | 30 |
| 80 − 30 | 50 |
Worked examples
91 − 48: 91 is about 90 and 48 is about 50. 90 − 50 = 40. The exact answer is 91 − 48 = 43, close to 40. The number line jumps back 40 and then 8.
| Number | About |
|---|---|
| 63 | 60 |
| 27 | 30 |
Hana has 84 beads. She uses 39. About 80 − 40 = 40 are left. Exactly 84 − 39 = 45 beads are left, so the estimate was close.
Class activity
Estimate first. Ask someone to write five subtraction problems with two-digit numbers on cards, such as 73 − 38. Round and estimate the answer in your head, say it aloud, then subtract exactly on paper. Which of your estimates was closest?
Practice
1
Estimate 59 − 21 by rounding to the nearest ten.
59 is about 60 and 21 is about 20, so 60 − 20 = 40.
2
Estimate, then subtract exactly: 72 − 28.
Estimate 70 − 30 = 40. Exact 72 − 28 = 44.
3
Salma says 83 − 47 = 56. Use an estimate to show that she is wrong.
83 − 47 is about 80 − 50 = 30. 56 is far from 30. The exact answer is 36.
4
A bakery makes 94 rolls and sells 58. About how many are left? Then find the exact number.
About 90 − 60 = 30. Exactly 94 − 58 = 36.
5
Round to the nearest ten: 46, 71 and 85.
50, 70 and 90.