I'm having alot of trouble with these homework problems and how to solve them.

Bob sets two alarm clocks (battery−powered) to be sure he arises for his Monday 8:00 A.M. accounting exam. There is a 73 percent chance that either clock will wake Bob. What is the probability that Bob will oversleep? If Bob had three clocks, would he have a 99 percent chance of waking up?

Half of a set of the parts are manufactured by machineAand half by machineB. Two percent of all the parts are defective. Four percent of the parts manufactured on machineAare defective. Find the probability that a part was manufactured on machineA, given that the part is defective.

I really appreciate if anyone could help me with these