In a department store, there were a certain number of red T-shirts, blue T-shirts, and yellow T-shirts. 3/7 of the T-shirts were red, 1/3 of them were blue and the rest were yellow. If there were 20 more red T-shirts than blue T-shirts, how many yellow T-shirts were there?

Answer 1

Let the number of T-shirts be
R, B, Y
R=(3/7)(R+B+Y)...(1)
B=(1/3)(R+B+Y)...(2)
Y=(1-3/7-1/3)(R+B+Y)...(3)
=(5/21)(R+B+Y)
R-B=20 ...(4)

Start with the last equation (4):
(3/7-1/3)(R+B+Y)=20
2/21(R+B+Y)=20
Solve for (R+B+Y), I get 210.

Substitute (R+B+Y)=210 into (1), (2) and (3) to get the number of t-shirts of each colour.