Ryan had three boxes, containing a total of 600 plastic plates. The number of plastic plates in Box A to the total number of plastic plates was 3:10. He sold 356 plastic plates from Box B and sold 1/4 of the plastic plates in Box C. The number of plastic plates left in Box B to the number of plastic plates left in Box C was 4:1. How many plastic plates were there in Box B at first?
a+b+c = 600
a/600 = 3/10
(b-356)/(3/4 c) = 4/1
now just solve for b and you get b=404