Answer
The required probability is, $\frac{1}{12}$
Work Step by Step
There are 36 equally likely outcomes; therefore, the number of elements in the sample is 36, that is, $ n\left( S \right)=36$
Assume $ E $ to be the event of obtaining two numbers which sum to 4; then $ E=\left\{ \left( 1,3 \right),\left( 2,2 \right),\left( 3,1 \right) \right\}$
Therefore, $ n\left( E \right)=3$
Thus, the probability of obtaining two numbers whose sums is 4 is:
$\begin{align}
& P\left( E \right)=\frac{n\left( E \right)}{n\left( S \right)} \\
& =\frac{3}{36} \\
& =\frac{1}{12}
\end{align}$
