The reason is because you multiply to get 8 and 40 to get the amount for regular time which is 320.Then you subtract 60 from 40 which is 20.You multiply that amounnt by 12 because that is the overtime which will equal 240.Add them together and you get 560. ☺
See attachment for one such shell. The volume is given by the sum of infinitely many thin shells like this, each with radius and height determined by the vertical distance between the upper blue curve and the lower orange curve for any given , i.e. .
What is the volume of a figure with that is 2 inches wide, 7 inches tall and 9 inchesn = Whereas the basic formula for the area of a rectangular shape is length × width, the basic formula for volume is length × width × height.