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Dad gives you money every day to put in your new piggy bank. He gives money to you in such a way that the money in the piggy bank doubles with each passing day.
If you already have 1 cent in the piggy bank and Dad gives you 1 cent the first day, 2 cents the second day, 4 cents the third day and so on, then your piggy bank gets full on the 14th day.
In addition to Dad's contributions, if Mom also gave you 1 cent the first day, 2 cents the second day, 4 cents the third day and so on, then on which day would your piggy bank be about half-full?
What is 12?
If both Mom and Dad contribute equal amounts to your piggy bank, then each needs to only make your piggy bank quarter-full. When both Mom and Dad contribute, the piggy bank will be half-full two days prior to the day it would be full when only Dad contributes. Thus, the piggy bank will be full on the 12th day when both Mom and Dad contribute.