Distinguish between group replacement and individual replacement.

The following failures have been observed for certain type of transistors in a digital computer:

The total number of transistors at the beginning of assembly is 1000 units. The cost of replacing the individual failed transistors is Rs. 1.25/-. The decision is made to replace all these transistors simultaneously at fixed intervals and to replace the individual transistors as they fail in service. If the cost of group replacement is 30 paisa per transistor, what is the best interval between group replacement?
 

  

End of Week Probability of failure to date 1 .05 2 .13 3 .25 4 .43 5 .68 6 .88 7 .96 8 00 1.00