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Recycling points

The recycling plant accepts glass, metal and plastic items. For every four items of the same type that I feed it I receive 1 green point, so I will receive 10 points whether I feed it 40, 41, 42 or 43 plastic items.

I fed it a 2-digit number of glass items, a larger 2-digit number of metal items and an even larger 2-digit number of plastic items. I received a 1-digit number of points for each type of item. The digits 1 to 9 each appeared only once among those 2-digit and 1-digit numbers.

If I told you the total number of items that I fed it you would be able to deduce with certainty how many of them were glass, how many were metal and how many were plastic.

How many were metal?

WIN £15 will be awarded to the sender of the first correct answer opened on Wednesday 27 May. The Editor’s decision is final. Please send entries to Enigma 1542, ¿ìè¶ÌÊÓƵ, Lacon House, 84 Theobald’s Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address).

Answer to 1536 List of primes: The sum of the prime numbers is 1287

The winner Bill Payne of Westfield, Surrey, UK