Question

If an AP has $a=1$,$t_n=20$ and $S_n=399$, then the value of n is

• A. 19
• B. 32
• C. 38
• D. 40

Answer 1

The correct option is C 38

Given, $a=1$

$t_n=1+(n-1)d=20$

$\therefore (n-1)d=19$

$S_n=\frac{n}{2}[2+(n-1\left)d\right]=399$

$\Rightarrow \frac{n}{2}[2+19]=399$

$\therefore n=38$