Using funding from Ontario's "Super build" Program, a contractor bought a box of decorative screws. Five of the screws are slotted, four are designed for a Phillips-head screwdriver, and three are designed for a square-tipped screwdriver. If two of the screws are randomly selected from the box, find the probability that they are both slotted.
a. Assume that the first screw is replaced before the second is selected.
b. Assume that the first screw is not replaced before the second is selected.