Filling Box

Sam keeps marbles in a box everyday in such a way that the number of marbles in the box doubles with each passing day. For e.g., he puts in 1 marble on the first day, 2 on the second day, 4 on the third day and so on. The box gets full on the 18th day. On which day was the box half full?

Answer on Monday.

Tags: brainteaser, logical puzzle