The question can be described in maths as the inequality:
(8+x)/10>2 where x>=0 because we cannot score negative goals. In other words, 8 goals from the previous five games plus an unknown amount of goals in the next 5 (x) divided by the number of games (10) should be greater than 2.
8+x>20 [Multiply each side by 10]
x>12 [Subtract 8 from each side]
He has to score more than 12 goals to have an average greater than 2.
Therefore, he must score at least 13 goals in the next five games to have an average goal per game greater than 2 (because our equation says he must score greater than 12, and we cannot score half goals so we round up to 13).