Cost of shirts = 4.Number of shirts + 20 or Cost = 4.S + 20

Cost of 10 shirts = 4.10 + 20 = 60
Cost of 20 shirts = 4.20 + 20 = 100
Cost of 30 shirts = 4.30 + 20 = 140
Cost of 40 shirts = 4.40 + 20 = 180

It fits with the notion that the cost increase is 40.