28 is not divisible by 3.
28 is divisible by: 1, 2, 4, 7, 14, 28.
28 is divisible by 1, 2, 4, 7, 14, 28.
20 is not divisible by 18.
18 is not divisible by 5 and 7.
138 is not divisible by 18.
19. All even numbers except 2 are divisible by 2 and so not prime - that eliminates 16, 18, 20, 28, 36 to leave 19. 33. 33 is divisible by 3 since 3+3 = 6 which is divisible by 3, leaving just 19 which is prime.
28 and its multiples.
The numbers that are divisible by 28 are infinite. The first four are: 28, 56, 84, 112 . . .
-- Neither 2 nor 5 is divisible by 18. -- 18 is divisible by 2, but not by 5.
28, 56, 84, and all other multiples of 28 that are factors of other numbers are also divisible by 28.
All multiples of 18 are divisible by 9. Not all multiples of 9 are divisible by 18.