A snail is at the bottom of a 20-foot well. Each day, it climbs up 3 feet, but at night, it slips back 2 feet. How many days will it take for the snail to reach the top of the well?
Correct: A
The snail effectively climbs 1 foot each day. On the 18th day, it will climb to the top of the 20-foot well. However, since the question asks how many days it takes to reach the top, and considering it climbs 3 feet on the last day and doesn't slip back because it's already out, we must calculate correctly: The snail needs to climb 20 feet. For the first 18 days, it climbs 3 feet and slips back 2 feet, making a net progress of 1 foot per day for 18 days, which equals 18 feet. On the 18th day (considering it as a full day of climbing without slipping back at night because it exits the well), it climbs the final 2 feet needed to reach or surpass the top. This thought process slightly misrepresents the exact mechanism; correctly, after 18 days of net gain (17 days of climbing and slipping, plus the final climb), it will be at 18 feet, and on the 18th climb, it will reach 21 feet, thus exiting.