Q1.
Add the following numbers. Wherever possible, find easier ways to add the pairs of numbers. (a) 15 + 79 (b) 46 + 99 (c) 38 + 35 (d) 5 + 89 (e) 76 + 28 (f) 69 + 20
Answer
All six answers first, then the easy way for each.
| Sum | Easy way | Answer |
|---|---|---|
| (a) 15 + 79 | 79 + 1 = 80, then 80 + 14 | 94 |
| (b) 46 + 99 | 46 + 100 = 146, then take 1 away | 145 |
| (c) 38 + 35 | 38 + 2 = 40, then 40 + 33 | 73 |
| (d) 5 + 89 | 89 + 1 = 90, then 90 + 4 | 94 |
| (e) 76 + 28 | 76 + 4 = 80, then 80 + 24 | 104 |
| (f) 69 + 20 | only tens change: 6 tens + 2 tens = 8 tens | 89 |
(a) 15 + 79 = 94
- 79 is close to 80. Give it 1 from the 15: 79 + 1 = 80.
- The 15 is now 14.
- 80 + 14 = 94.
(b) 46 + 99 = 145
- 99 is 1 less than 100. Add 100 instead: 46 + 100 = 146.
- You added 1 too much, so take 1 back: 146 − 1 = 145.
(c) 38 + 35 = 73
- 38 needs 2 to reach 40. Take 2 from the 35: 38 + 2 = 40.
- The 35 is now 33.
- 40 + 33 = 73.
(d) 5 + 89 = 94
- 89 needs 1 to reach 90. Take 1 from the 5: 89 + 1 = 90.
- The 5 is now 4.
- 90 + 4 = 94.
(e) 76 + 28 = 104
- 76 needs 4 to reach 80. Take 4 from the 28: 76 + 4 = 80.
- The 28 is now 24.
- 80 + 24 = 104.
(f) 69 + 20 = 89
- 20 is 2 whole tens, so only the tens digit changes.
- 6 tens + 2 tens = 8 tens; the 9 ones stay as they are.
- So the answer is 89.
Why these shortcuts are safe: You are only moving a few ones from one number to the other. Nothing is lost and nothing is added, so the total stays exactly the same — but one of the numbers becomes a friendly 10, 20, 40 or 80, and those are easy to add in your head.
Check it yourself: Do (e) by columns as well. Ones: 6 + 8 = 14, write 4 carry 1. Tens: 7 + 2 + 1 = 10, write 0 carry 1. Hundreds: 1. That gives 104 — the same answer.