Find all least nonnegative incongruent solutions of the following congruences. (a) 9x ≡ 21 (mod 30)
a) x ≡ 3 (mod 10)
b) x ≡ 13 (mod 10)
c) x ≡ 23 (mod 10)
d) x ≡ 33 (mod 10)