math !!!! :O
posted by Katherine .
In a department store, there were a certain number of red Tshirts, blue Tshirts, and yellow Tshirts. 3/7 of the Tshirts were red, 1/3 of them were blue and the rest were yellow. If there were 20 more red Tshirts than blue Tshirts, how many yellow Tshirts were there?

Let the number of Tshirts be
R, B, Y
R=(3/7)(R+B+Y)...(1)
B=(1/3)(R+B+Y)...(2)
Y=(13/71/3)(R+B+Y)...(3)
=(5/21)(R+B+Y)
RB=20 ...(4)
Start with the last equation (4):
(3/71/3)(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 tshirts of each colour.