Find the missing number in the following series.
1, 6, 15, (…) , 45, 66, 91

Correct option is A

Let's analyze the given series:
1, 6, 15, (…), 45, 66, 91
Step 1: Find the differences between consecutive terms:
6−1=5
15−6=9
(missing number)−15=?
45−(missing number)=?
66−45=21
91−66=25
We observe that the differences are increasing by 4 each time:
5, 9, ?,?, 21, 25.
Step 2: Predict the missing difference:
The next difference should be 9+4=13 and 13 + 4 = 17
Step 3: Calculate the missing term:
Missing – 15 = 13
Missing = 15 + 13 = 28
Thus, the missing number in the series is 28.