Please also provide an explanation for each step

Q\#1 Design I or Design II - Class Problem (Note: data values in Q# 1-Design I and Design II and Q#2 below are different) Two designs are under consideration for manufacturing. Design I has two circuits A and Bin series with the warrantee period reliability 0.9,0.9 respectively. Design II has also two circuits C and D, but in parallel with reliability 0.8,0.8 respectively. Costs of repairing circuits A,B,C,D are $15,$10,$30, and $25 respectively. (a) Under the policy of replacing all defective circuits of a product during the warrantee period, which design is preferable for minimizing the total repair cost? (Are any ethics questions here?) (b) If Design I is used and 100 units of the product are sold; on the average, how many units can be repaired by replacing only the circuit B