Exercise
Which of the following statements is true in (Z/35Z,·,1)?
7a = 7 implies a = 1
15a = 15 implies a = 1
22 a = 22 implies a = 1
No, 7 · 6 is also 7.
No, 15 · 8 is also 15.
Indeed, we can apply the cancellation law since 22 is invertible.