We need to find the number of tie-dyed t-shirts, for that we will assign a variable for each kind of shirt. The tie-dyed will be called "x" and the silk-screened will be called "y".
We know that he sold 180 shirts in total, so the sum of x and y should be 180.
We also know that he sold 4 times as many tie-dyed t-shirts as silk-screened ones, so:
To solve the problem we need to replace the second equation on the first one:
Then we can use the second equation to find the value of x.
He sold 144 tie-dyed shirts and 36 silk-screened ones.