IKMC 2019 - Grades 11-12 - Set 2 - Self-study Question 29

Question 29

Three different numbers are chosen at random from the set $\{1,2,3,\ldots,10\}$. What is the probability that one of them is the average of the other two?

$1/10$
$1/6$
$1/4$
$1/3$
$1/2$