The average (arithmetic mean) of A and B is 10, whereas the average of B and 10 is C/2. What is the average of A and C?
This looks like a SAT question.
$\displaystyle \frac{A+B}{2}=10$
$\displaystyle \frac{B+10}{2}=\frac{C}{2}$
Thus,
$\displaystyle A+B=20$ (1)
$\displaystyle B+10=C$ (2)
From (1) subtract (2),
$\displaystyle A-10=20-C$
$\displaystyle A+C=30$
$\displaystyle \frac{A+C}{2}=15$