A1. The average of the numbers $2,5, x, 14,15$ is $x$. Determine the value of $x$.
Correct answer: 9 .
Solution 1: The average of the numbers is $\frac{2+5+x+14+15}{5}=\frac{x+36}{5}$; this must be equal $x$. Solving $36+x=5 x$, we have $x=9$.

Solution 2: Since the $x$ being there would not affect the average of the other 4 numbers, $x=\frac{2+5+14+15}{4}=\frac{36}{4}=9$.