Question

Insert the missing number in the series.

1, 3, 6, 10, 15, …, 28, 36, 45 : 

(a) 17 

(b) 19 

(c) 21 

(d) 22 

Answer 1

Correct option (c) 21

Explanation:

The differences of each successive and previous numbers has the sequential increase of one as :3 − 1 = 2, 6 − 3 = 3, 10 − 6 = 4, 15 − 10 = 5, 21 − 6 = 15, 28 − 7 = 21, 36 − 8 = 28, 45 − 9 = 36, (i.e. 2, 3, 4, 5, 6, 7, 8 and 9). 

Therefore, the missing number is 21.