The mean time it takes for a particular makemodel of a watch

The mean time it takes for a particular make/model of a watch to break down and require repair services from the manufacturer is 30 months, with a standard deviation of 8 months. The manufacturer

Solution

Mean =30 and std devition = 8

Prob of service under warrante <=0.03

i.e. |z| >2.17

x should lie between 30-8(2.17),30+8(2.17)

i.e. x should be < 47.36

Thus warranty period should be fixed as 47.36 months or 4 years .