Acertain drug is made from only two ingredients. compound a and compound b. there are 5 millimeters of compound a used for every 6 millimeters of compound b. if a chemist wants to make 682 millimeters of the drug how many millimeters of compound b are needed?

A certain drug is made from only two ingredients: compound A and compound B. There are 5 milliliters of compound A used for every 4 milliliters of compound B. If a chemist uses 255 milliliters of compound A, how many milliliters of the drug will be made? 

4:5 is the same as 4x:5x where 4x is compound A and 5x is compound B.
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Solve for "x":
If amount of compound A is 4x = 255

Idon't take stuff like this new to me