Acertain shade of green requires 4 parts blue for every 5 parts yellow. if you have 10 parts blue, how many parts yellow are needed to produce the same shade of green?

You would approach this problem by setting a proportion. Firstly, the ratio to get the certain shade of green would be 4 : 5 blue to yellow. You have 10 parts blue, so lets say x parts yellow.
The proportion is
4/5 = 10/x
We would then solve for x. Cross multiply
50 = 4x
x = 12.5 parts yellow
I hope this helps :-)
we can check to see if the two ratios are the same
4/5 = .8
10/12.5 = .8