An electronic equipment contains 1000 resistors. When any resistor fails, it is replaced. The cost of replacing a resistor individually is ! 10.00. If all the resistors are replaced at the same time, the cost per resistor is ! 4.00. The per cent surviving, S(i) at the end of month i is given below. What is the optimum replacement plan?

i 0 1 2 3 4 5 6 S(i) 100 96 89 68 37 13 0 AppendixLO1