The probability of a man hitting a target in one fire is $\frac{1}{5}$. Then the minimum number of fire he must follow in  order to make his chance of hitting the target more than $\frac{3}{4}$is

 Probability of hitting the target in one fire, $p=\frac{1}{5}$ Then the probability of hitting the target at least once in n fires

$\begin{array}{l}=1-\text{ (probability of not hitting the target) }\\ =1-(4/5{)}^n>3/4\\ \Rightarrow (4/5{)}^n<1/4\\ \Rightarrow \text{ The least value of }n\text{ is }7\end{array}$