Brain Teasers 56 - Puzzle Prime

Burn the Ropes

Section: Brain Teasers Category: Insight

Difficulty: Advanced

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?

SOLUTION

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.

Unknown Author February 1, 2017

2 Comments

Running Dog

Section: Brain Teasers Category: Insight

Difficulty: Medium

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?

SOLUTION

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.

Unknown Author January 16, 2017

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Missionaries, Cannibals

Section: Brain Teasers Category: Deduction

Difficulty: Advanced

Three missionaries and three cannibals must cross a river with a boat which can carry at most two people at a time. However, if on one of the two banks of the river the missionaries get outnumbered by the cannibals, they will get eaten. How can all 6 men cross the river without anybody gets eaten?

Remark: The boat cannot cross the river with no people on board.

SOLUTION

Label the missionaries M1, M2, M3 and the cannibals C1, C2, C3. Then:

1. M1 and C1 cross the river, M1 comes back.
2. C2 and C3 cross the river, C2 comes back.
3. M1 and M2 cross the river, M1 and C1 come back.
4. M1 and M3 cross the river, C3 comes back.
5. C1 and C2 cross the river, C1 comes back.
6. C1 and C3 cross the river.

Now, everyone is on the other side.

Unknown Author January 7, 2017

1 Comment

Friends and Enemies

Section: Brain Teasers Category: Deduction

Difficulty: Advanced

Show that in each group of 6 people, there are either 3 who know each other, or 3 who do not know each other.

SOLUTION

Let’s call the people A, B, C, D, E, F. Person A either knows at least 3 among B, C, D, E, F, or does not know at least 3 among B, C, D, E, F.

Assume the first possibility – A knows B, C, D. If B and C know each other, C and D know each other, or B and D know each other, then we find a group of 3 people who know each other. Otherwise, B, C, and D form a group in which no-one knows the others.

If A doesn’t know at least 3 among B, C, D, E, F, the arguments are the same.

Unknown Author January 4, 2017

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