1
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Question

# Given that 1 + 2 + 3 + 4 + 5 + 6 = 21, the sum of 61 + 62 + 63 + 64 + 65 + 66 is .

A
351
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B
341
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C
361
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D
381
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Solution

## The correct option is D 381We can use the method of smart addition to easily find out the sum. We can splt up the numbers in the sum of 61 + 62 + 63 + 64 + 65 + 66 as (60 + 1), (60 + 2), ... , (60 + 6). Then grouping the 60 together, 61 + 62 + 63 + 64 + 65 + 66 becomes 60 × 6 + (1 + 2 + 3 + 4 + 5 + 6) Since it's already given that 1 + 2 + 3 + 4 + 5 + 6 = 21, Sum = 60 × 6 + 21 = 360 + 21 = 381

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