A3. The arithmetic mean of the numbers $2,4,5,8, x, 15,20$ is $x$. The value of $x$ is:
(A) -9
(B) 9
(C) 10
(D) 12
(E) 13
A3. Write the equation $\frac{2+4+5+8+x+15+20}{7}=x$, which has the solution $x=9$.