If each shirt costs $10 within the purchase of 1 to 75 shirts, the function will be: 10x (with x being the number of shirts, from 1 to 75). If each shirt costs $7.50 within the purchase of 76 to 150 shirts, the function will be: 7.5x (with x being between 76 and 150). Finally, if each shirt costs $6 within the purchase of 150 or more shirts, the function will be: 6x (with x being any number from 150 up).