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Age 11 to 14 Short
Challenge Level

Answer: 32

First round:       only even numbers left

Second round:         only multiples of 4 left

Third round:         only multiples of 8 left

Fourth round: Only multiples of 16 will be left

Fifth round: Only multiples of 32 will be left.
                  32 is the only multiple of 32 under 50, so 32 is the last number left.

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.