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.

if the perimeter of one is 40, then the lengths can be evenly divided among the four sides.

two of the four sides of each rectangle making up the perimeter are showing, meaning that half of each rectangle is to be accounted for.

so 20 cm for each rectangle

so the perimeter of ABCD should be 80cm^2