1
Guess the sum first
Discover Mathematics P2: Term 1, Lessons 31–40
GoalEstimate a sum by adding the tens digits, then add exactly and check that the two are close.
New words
estimate · a careful guess that is close to the answertens digit · the digit that tells how many tens a number hassum · the answer when we addclose · not far from the real answer
Explanation
Before you add, make a good guess. Look only at the tens digits. In 36 + 23 the tens digits are 3 and 2. That means about 30 and about 20. Now 30 + 20 = 50, so the sum is about 50. An estimate is not exact. Next add the real numbers: 36 + 23 = 59. The real sum is close to 50, a little bigger, because the ones add some more.
| Number | Tens digit | About |
|---|---|---|
| 36 | 3 | 30 |
| 23 | 2 | 20 |
| Guess: 30 + 20 | 50 |
Worked examples
Hana has 52 stickers and Omar has 35. Estimate with the tens digits: 50 + 30 = 80. The real sum is 52 + 35 = 87, which is close to 80.
| Number | Tens digit |
|---|---|
| 28 | 2 |
| 41 | 4 |
We can also estimate a take-away. For 68 − 23 think 60 − 20 = 40. The real answer is 45, close to 40.
Class activity
Number cards. Write the numbers 11 to 49 on small papers and put them in a bag. The child pulls two papers, says the tens digits, and guesses the sum aloud. Then the adult checks with the real sum. If the guess is within about 10 of the real sum, the child scores a point.
Practice
1
Estimate 41 + 26 with the tens digits.
40 + 20 = 60, so about 60.
2
Youssef has 34 marbles and wins 25 more. Estimate how many he has now.
30 + 20 = 50, so about 50. (The real number is 59.)
3
Estimate 72 − 31 with the tens digits.
70 − 30 = 40, so about 40. (The real answer is 41.)
4
Seif says 53 + 24 is about 70. Fill in the tens digits and decide if he is right.
| Number | Tens digit |
|---|---|
| 53 | ? |
| 24 | ? |
Tens digits 5 and 2: 50 + 20 = 70. Yes, he is right.
5
Estimate first, then add exactly: 45 + 32.
Guess 40 + 30 = 70. Real sum 77.