self Maths Puzzle

For which integers n>1 does the set of positive integers less than and relatively prime to n constitute an arithmetic progression?

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self Other Question

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? Compare the numbers 0.99999... (infinitely many 9s) and 1. Which of the following statements is true? Why?

0.99999 ... < 1
0.99999 ... = 1
0.99999 ... > 1