Answer
$\dfrac{1}{6}$
Work Step by Step
We know that probability of an event $E$ is given by the formula:
$P(E)=\dfrac{\text{number of favorable outcomes}}{\text{number of all outcomes}}$
The generalized basic counting principle says that if an event $e_1$ can be performed in $n_1$ different ways and an event $e_2$ can be performed in $n_2$ different ways, then there are $n_1\cdot n_2$ different 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 $6\cdot6=36$ possible outcomes.
Out of these, the outcomes that have a sum of $7$ are:
$(1,6),(2,5),(3,4),(4,3),(5,2),(6,1).$ ($6$ favorable outcomes)
Thus,
$P(\text{sum is 7})=\dfrac{6}{36}=\dfrac{1}{6}$
