18. The cost of a unit is made up of $6.25 material cost, $4.75 labour cost, and $3.25 overhead. What is the ratio that exists between the three elements of cost? 19. Solve: a:6 =20:40 20. Material cost of a computer is five-eights of total cost, and labour cost is one-third of material cost. If labour cost is $20, what is the total cost of the computer? 21. Your utility bill for April is $170. If you pay after the due date, a late payment penalty of $7.72 is added. What is the percent penalty? 22. A brokerage firm charges a fee of 1 X%. If its fee on a stock purchase was $400, what was the amount of the purchase