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
