The enterprising snail

A group of snails had been living happily for many years at the bottom of a twenty foot well when it suddenly ran dry. Most of the snails lay down to did, but one industrious fellow decided to in elsewhere. The next dawn he began to climb to the top of the well at the prodigious rate of three inches per hour for each of the twelve daylight hours. Each night, however, he slipped back at the rate of two inches per hour. How long did it take this enterprising snail to escape from the well?

Solution :
17 1/2 days. The snail climbed at the net rate of one foot per day (3-2=1). At dawn of the start of the eighteenth day he was, therefore, 17 feet from the bottom or 3 feet or 3 feet from the top. Since he climbed 3 feet during the twelve daylight hours of the eighteenth day he would reach the top at sunset of the eighteenth day, or 17 1/2 days after he started his climb. -QED-