Question

If first term of an AP is 5, last term is 45 and the sum of the terms is 400, then the number of terms is in the AP is ___.

• A. 8
• B. 10
• C. 16
• D. 20

Answer 1

The correct option is C

16

Here, first term $\left(a\right)=5$
last term $\left(l\right)=45$, and $S_n=400$ where $n^{th}$ term is the last term.

$\because S_n=\frac{n}{2}[a+l]$

$\Rightarrow 400=\frac{n}{2}(5+45)$

$\Rightarrow n=16$

$\therefore$ Number of terms is in the AP = 16