Well, if you have 3/5, and the rest are yellow, this results yellow being 2/5
But, if you multiply it, (2/5 x 2=4/10) you’re going to have to multiply the green marbles by 2, too. Yes, it’s a different fraction, but they are equivalent fractions. So, dividing by them both by 2 will give you the same amount you begun with.
In general, the fraction of yellow will be 4/10.
Hope this helps!