Blue shirts = B
Yellow shirts = Y
Total shirts = T

For every 3B there are 5Y

Then we clear a variable of one of the equations:
In 3B = 5Y

We use this in the equation T = B + Y with T=72

We clear the Y:



We use this value of Y in the equation 3B=5Y to know the B:


Then so, the number of shirts is:27 yellow shirts and 45 Blue shirts