self Maths Puzzle

The sum of N real numbers (not necessarily unique) is 20. The sum of the 3 smallest of these numbers is 5. The sum of the 3 largest is 7. What is N?

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self Other Question

21!=510909x21y1709440000

Without calculating 21!, what are the digits marked x and y? explain.
For which integers n>1 does the set of positive integers less than and relatively prime to n constitute an arithmetic progression?