🔴 hardlogic

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?

Illustration for riddle: A snail is at the bottom of a 20 meters deep pit. Every day the snail climbs 5 meters upwards, but a

The snail reaches the top of the pit on the 16th day!

Why this works

This riddle plays with the idea of net progress over time. Each day, the snail effectively climbs only 1 meter after accounting for the slide back down (5 meters up minus 4 meters down). However, on the 16th day, when the snail climbs 5 meters, it reaches the top of the 20-meter pit and doesn't slide back down because it has already exited the pit. So, while it seems like the snail would take much longer, it actually only takes 16 days to reach the top!

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Common Wrong Answers

“20 days”

This guess assumes the snail climbs the entire 20 meters without considering the nightly slide, which is not how the riddle operates.

“15 days”

This guess underestimates the snail's progress by not accounting for the last day when the snail climbs out of the pit before sliding back.

“10 days”

This guess is too low and does not take into consideration the daily net gain of only 1 meter (5 meters up minus 4 meters down) until the final climb.

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