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In the following systems Z_n, write down the sets of elements that have inverses. ( (n) is equal to the number of elements in these sets.)

a) Z_7

d) Z_13

The answers to 'a' and 'd' are 4 and 12 respectively.

The question asks for "the sets of elements that have inverses" but 4 and 12 are just numbers, so how can they be the answers to the questions? I learned that if a number n(bar) is in z_m then n(bar) has a multiplicative inverse iff gcd(m,n) = 1 but I'm not sure how to apply this here. Can someone help me with this?