suppose a group of 30 identical machines is monitored for 6 months and without PM , the number of breakdowns in each month is as follows;

month of operation before breakdown, n 1 2 3 4 5 6

no of break downs, b ----------------------- 3 5 5 4 3 10

This indicated the longest time a machine runs before breaking down in 6 months and that for example, 5 machines ran 2 months before they broke down. Cost of repairing the machine after breakdown is Rs 1000(CB), Where as cost of PM on a machine is Rs 200(,PM Cp).PM does not eliminate break downs.

Calculate

1) what is the monthly cost of breakdowns

2) if PM used , How often should it be done