J's answer to Fiona's Primary 5 Maths Whole Numbers Singapore question.
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1 + 2 + 3 + 4 + ... + 137 + 138 + 139 + 140
You can pair the numbers like this :
1 + 140 = 141
2 + 139 = 141
3 + 138 = 141
4 + 137 = 141
...
...
68 + 73 = 141
69 + 72 = 141
70 + 71 = 141
There are 140 numbers , so we can form 70 such pairs. (140 ÷ 2 = 70)
Sum = 141 × 70 = 9870
You can pair the numbers like this :
1 + 140 = 141
2 + 139 = 141
3 + 138 = 141
4 + 137 = 141
...
...
68 + 73 = 141
69 + 72 = 141
70 + 71 = 141
There are 140 numbers , so we can form 70 such pairs. (140 ÷ 2 = 70)
Sum = 141 × 70 = 9870
Date Posted:
2 years ago
Thx.
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