A snail is at the bottom of a 20 meters deep pit. Every day the snail climbs 5 meters upwards, but at night it slides 4 meters back downwards. How many days does it take before the snail reaches the top of the pit?
-
21 days
-
19 days
-
16 days
-
17 days
C
Correct answer
Explanation
The snail climbs net 1 meter per day (5 up, 4 down). After 15 days: 15 meters reached. On day 16, climbs 5 meters from 15 to reach 20 meters and exits - doesn't slide back. The trick is that on the final day, the snail reaches the top before nightfall. Common trap: dividing 20/1 = 20, but this ignores the final ascent.