The mean is the sum of the values divided by the amount of values.
So the mean is represented here by
\(\frac{12+17+15+x}{4}=13\)
Solve for \(x\):
\(\frac{12+17+15+x}{4}=13\)
\(\rightarrow\frac{44+x}{4}=13\)
\(\rightarrow 44+x=52\)
\(\rightarrow x=8\)
