Question

A clock strikes once at 1 o'clock, twice at 2 o'clock, thrice at 3 o’clock and so on. How many times will it strike in 12 hours?

• A. 156
• B. 78
• C. 132
• D. 66

Answer 1

The correct option is B

78

The total number of strikes $(1+2+3+\dots +12)$
The sum of first n terms of an AP: $S=\left(\frac{n}{2}\right)[2a+(n-1\left)d\right]$
where, n = 12, a = 1, and d = 1
$S=\left(\frac{12}{2}\right)\left[2\right(1)+(12-1\left)1\right]=\left(6\right)[2+11]=78$