Question

The first two terms of an A.P. are $27$ and $24$ respectively. How many terms of the progression are to be added to get $-30$?

• A. $15$
• B. $20$
• C. $25$
• D. $18$

Answer 1

The correct option is B $20$

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

$-30=\frac{n}{2}[2\times 27+(n-1\left)\right(-3\left)\right]$

$-30\times 2=3n[19-n]$

$20\times -1=n[19-1]$

$n=20$