The integers 1,2,3,4,......n are written on a blackboard. The following operation is then repeated until we will get only one number: In each repetition, any two numbers say a and b, currently on the blackboard are erased and a new number a + b is written. But one number was missed by mistake. The sum obtained was 1525. Which number was missed?

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Solution

The correct option is A
15

$\frac{n (n + 1)}{2} = 1525; n (n + 1) = 3050; n = 55$ (approx.)
$\frac{55 \times 56}{2} = 1540;$ missed number = 1540-1525 = 15.

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The correct option is A 15 n(n+1)2=1525; n(n+1)=3050; n=55 (approx.) 55×562=1540; missed number = 1540-1525 = 15.

The correct option is A
15

$\frac{n (n + 1)}{2} = 1525; n (n + 1) = 3050; n = 55$ (approx.)
$\frac{55 \times 56}{2} = 1540;$ missed number = 1540-1525 = 15.