Question

The positive integers are arranged in a continuation of the pattern shown below. The value of the integer in the 72nd row (horizontal) and the 15th column (vertical) is
A) 2402 B) 3670 C) 3688 D) 3472 E) 2416

Pattern:
1 3 4 10 11
2 5 9 12
6 8 13
7 14
15

Answer 1

first, note that the element at (72,15) is the 15th element along the diagonal starting at row 86, which starts at (86,1) and moves up to the right.

the even-numbered rows start with 1 + the previous odd-numbered row
The odd-numbered row n starts with the value n(n+1)/2

so the element at (86,1) is 1 + 85*86/2 = 3656
so the element at (72,15) is 3656 + 14 = 3670

Answer 2

Hey, oobleck
I don't understand your explanation can you elaborate?

Answer 3

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