The ratio would be 6:10 or 6/10 ( 3/5) or 0.6. Why? A ratio is comparing two numbers with a fraction, colon or decimal or percent.
Well there are 6 blue marbles out of the 35 colored marbles. And there are 10 clear marbles.
The question is asking for a blue/clear. It is easy to get confused and put clear/blue. So that is why 6:10 is the ratio.