Clue 1: Only numbers between 0 – 20
Clue 2: A’s number is either 5, 10, 15 or 20
Clue 3: C : B = 2 : 1
Clue 4: D : C = 2 : 6
Clue 5: C > B > A
Clue 6: D is not the smallest

Looking at Clues 3 and 4, we can combine the ratios as below:

B : C : D
3 : 6 : 2

Since D cannot be the smallest number, we can multiply it by 3 to get 6, which is larger than the first multiple of 5, which is 5 itself. The ratio becomes:

A: B : C : D
5 : 9 : 18 : 6