13. The mean of $p$ and $q$ is 13 ; the mean of $q$ and $r$ is 16 ; the mean of $r$ and $p$ is 7 . What is the mean of $p, q$ and $r$ ?
A 12
B 13
С 14
D 15
E 16
13. A The mean of $p$ and $q$ is 13 . Therefore $p+q=2 \times 13=26$. Similarly, $q+r=2 \times 16=32$ and $r+p=2 \times 7=14$. Adding these three equations gives $2 p+2 q+2 r=26+32+14=72$. Hence $p+q+r=36$. So the mean of $p, q$ and $r$ is $\frac{p+q+r}{3}=\frac{36}{3}=12$.