Question

The sum of $n$ terms of a $GP$ is $381$. Its first term is $3$ and common ratio is $2$.Find the number of terms and

Answer 1

$381=\frac{3(2^n-1)}{2-1}127=2^n-1128=2^n{2}^7=2^{n}n=7$

mo. of terms are $7$