Only possibility I can think of is 15 + 15 = 30 i.e. if we can leave one box empty as they said we can repeat the numbers but did not mention that we need to fill all 3 boxes.
Thank You Sir! A lil bit more out of the box could also be to say "No, we cannot solve this" in response to "Can you Solve this?"
I am not that old either sir. I think you have mistaken me with somebody else with the same name -rgs