The ratio of blue marbles to green marbles if there are 10 blue marbles for every 3 green marbles

84.

I didn’t know how to solve this properly, so there’s probably a much more elegant way to figure this out, but this is what I came up with:

First, I added 5, 7 and 8 together to get 20. So, in a group of 20 marbles, 7 of them will be green.

I divided 240 by 20 to get 12, meaning there were 12 groups of 20 total. Since there were 12 groups, that means you could multiply 12 by 7 (since 7 is the number of green marbles in a group) to get the total number of green marbles. 12 times 7 is 84.