If the fifth term of a GP is 81 and first term is 16, what will be the 4th term of the GP?

A. 36 B. 18 C. 54 D. 24 Answer: Option C
\begin{aligned}&5^{th}\text{ term of GP }\\ &=ar^{5-1}\\ &=16\times r^4\\ &=81\\ &\text{ Or, }r=\left(\frac{81}{16}\right)^{\frac{1}{4}}=\frac{3}{2}\\ &4^{\text{th }}\text{ term of GP }\\ &=ar^{4-1}\\ &=16\times\left(\frac{3}{2}\right)^3\\ &=54\end{aligned}