1.  An intelligent boy is doing a jigsaw with 275 pieces. Each day that he fits pieces together there are fewer pieces left, and it is reasonable to assume that he fits more pieces each day because the number left to sort out diminishes progressively. Hence he is able to fit an extra piece as each day goes by. On the first day he fits 20 pieces. How many days does it take for him to complete the puzzle ?
A. 11 days
B. 15 days
C. 13 days
D. 16 days

Answer: Option A

Explanation:

Let 'n' number of days it take to complete.

we know that sum of n number= n/2[2a+(n-1)d]

where a=20,d=1;

n/2[ 2 x 20 + (n-1)1 ]=275

n[n+39]=550

+ 39n - 550 = 0

+ 50n - 11n - 550 = 0

(n-11) (n-50) = 0

n-11 = 0;

n = 11(Days).