Help needed on Tricky Ratio Question

[kopper]

Marbles are coloured Red, Green, Yellow and Blue in the ratio 9:5:3:1 respectively. Why can't there be 7 yellow marbles?

[mad?]

The ratio of blue to yellow is 1:3, so 2:6 or 3:9, the number of yellow marbles has to be a multiple of 3.

[kopper]

The ratio of blue to yellow is 1:3, so 2:6 or 3:9, the number of yellow marbles has to be a multiple of 3.

Thanks