# Brainteasers

Triangle puzzles

In this triangle, a number in a square is the sum of the two numbers on each side of it.

Complete these two triangles.

Passing trains

Two trains of equal length pass each other in opposite directions.

Each train is travelling at 72kmh and is of the same length. The driver of one train finds that it takes the other train exactly 4 seconds to pass him.

How long is each train?

Passing trains (age 14-16): 160 metres. Let s be the length of a train in km. Relative to the other train, one of the trains must travel a distance s at 144kmh to pass it completely, which it does in 4 seconds. Since distance = speed x time: s = 144 x 4 x 1,000m = 160 metres 60 x 60

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