Originally Posted by Safin

How terrible. Not even a single attempt.

Anyway, use the principle of base. Here is an explanation.
value of 15 is 15 because its base 10.
that is
(10^1)*1+(10^0)*5
(ten raised to 1 is 10 and ten raised to 0 is 1)

So in this case, whenever you see such kind of puzzles its usually a base change problem

So we have at base 18
converting to decimal
(18^1)*1+8 (as 8 remains as such) = 26
Half of that is 13.

Caveat: If someone would have solved this you could have said the answer as 21 which is also logically right as there is no distinguishing method to know if 13 is in decimal or base 18.