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We know that every 2nd biscuit has icing. These are the even numbers (2, 4, 6, 8, 10, 12, 14, 16, 18, 20)
We know that every 3rd biscuit has a cherry. These are multiples of 3 (3, 6, 9, 12, 15, 18)
we know that every 4th biscuit has a choc button. These are multiples of 4 (4, 8, 12, 16, 20)

There are 6 other biscuits with no decorations (5th, 7th, 11th, 13th, 17th, 19th)

The 12th biscuit has all decorations because it's an even number that is a multiple 3 and 4.