Correct Answer - Option 3 : 12

__Concept__:

The sum of n terms of an AP with first term a and common difference d is given by:

\(\rm S_n= \frac{n}{2}×(a+l)\) or \(\rm S_n= \frac{n}{2}×\left [ 2a+(n-l) \right ]\)

Where l is the last term of the AP

__Calculation:__

Given: First term of AP = a = 15

Last term of AP = l = 60

Sum of n terms of an AP = 450

⇒ \(\rm S_n= \frac{n}{2}×(a+l)\)

⇒ 450 = n/2 × (15 + 60)

⇒ 6 = n/2

**∴ n = 12**