Answer
$\dfrac{1}{18}$
Work Step by Step
We know that probability$=\dfrac{\text{number of favourable outcomes}}{\text{number of all outcomes}}.$
The generalized basic counting principle says that an event $e_1$ can be performed in $n_1$ ways and an event $e_2$ can be performed in $n_2$ ways, then there are $n_1n_2$ ways of performing them together. This can easily be extended to $n$ events.
Since a die has $6$ sides with $6$ different numbers, then there are $6$ possible outcomes when one die is rolled.
Hence, when two dice are rolled, there are $6⋅6=36$ possible outcomes.
Out of thesethe outcomes that have a sum of $3$ are: $(1,2),(2,1).$ ($2$ favourable outcomes).
Thus,
$P(\text{sum is 3})=\dfrac{2}{36}=\dfrac{1}{18}$
