A snail is at the bottom of a 100 km deep pit. Each day he climbs up 5 km, but at night, he slips down 1 km. How many days does it take him to reach the top?

Answer: D. 25days

A snail is at the bottom of a 100 km deep pit. Every day it climbs 5 km but slips back 1 km at night, so its net progress in one full day is 4 km. However, on the last day it will not slip back because it reaches the top. After 24 days the snail reaches 96 km (24 × 4). On the 25th day it climbs 5 km and reaches 101 km, which is above 100 km. Therefore, the snail reaches the top in 25 days.

Answering after opening this box earns no Practice Points.