Q:

How to solve this problem on probability? 

The probability of a man hitting a target is 3/4.  He tries 5 times.  What is the probability that the target will be hit at least 3 times ? 

A:

Hint.  According to this LINK, the probability that after $5$ tries, the man hit the target exactly $k$ times is
$$\binom{5}{k}p^k(1-p)^{5-k}$$
where $p=3/4$. Can you take it from here?