Two persons A and B have equal number of sons. There are three cinema tickets which are to be distributed among the sons of A and B. The probability that all the tickets go to sons of A is $\large\frac{1}{20}$. The number of sons each of them is :

Question

$\begin {array} {1 1} (1)\;3 & \quad (2)\;4 \\ (3)\;5 & \quad (4)\;6 \end {array}$