Select all ratios equivalent to 5: 15.
2: 60, 20: 9, 7: 21, 1: 3

Well ratios involve a relationship between two numbers,
5:15 when divided by 5 (because 5 is the biggest number that fits In both halves.
Equals

1:3
And this ratio is the same as any ratio where the left number fits into the right three times, and in absolutely no other order. for example,
2:6
3:9
4:12
5:15
6:18
7:21 and so on...