Last Week's POW

 The Boy and His Donkey

One way to solve the problem is to work backwards.  After his last trip he gave the donkey $24,  which was all that he had after the donkey had doubled his money.  So before the donkey doubled his money he must have had $12.  This was after the donkey had taken $24 after his second crossing, which means he must have had $36 after his money was doubled after his second crossing.  Therefore he had $18 before he crossed the second time.  This was after the donkey had taken $24 for his first crossing, so he must have had $42 after crossing the first time having doubled $21, the amount he started with.  To check start with $21. Cross the bridge once and you have 2 X $21 or $42.  The donkey takes $24, leaving you with $18.  Cross the bridge a second time and your money doubles to 2 X $18 or $36. The donkey takes $24, leaving you with $12.  Cross the bridge a third time and your money doubles to 2 X $12 or $24.  the donkey takes his $24 and you are left with nothing.