Snail on a Tree

A snail is trying to climb a 10-meter poll. Every day it climbs 4 meters up and then during the night slides 3 meters down. How many days are needed for the snail to get to the top of the poll?

Seven days only. After the sixth day, the snail would have climbed 6 meters. During the seventh day, it will climb 4 more meters and will get to the top.

Burn the Ropes

You have two ropes and a lighter. Each of the ropes burns out in exactly 60 minutes, but not at a uniform rate – it is possible for example that half of a rope burns out in 40 minutes and the other half in just 20. How can you measure exactly 45 minutes using the ropes and the lighter?

First, you light up both ends of the first rope and one of the ends of the second rope. Exactly 30 minutes later the first rope will burn out completely and then you have to light up the other end of the second rope. It will take 15 more minutes for the second rope also to burn out completely, for a total of 30 + 15 = 45 minutes.

Running Dog

Two people – Mick and Goof, 100 meters apart, start walking towards each other with constant speeds of 2m/s. At the same time Mick’s dog starts running back and fourth between them with constant speed of 6m/s until Mick and Goof meet. How much distance does the dog cover in total?

Mick and Goof will meet after 100/(2 + 2) = 25 seconds. Therefore the dog will run for 25 seconds and will cover 6 x 25 = 150 meters in total.