A sample data of 20 failures (time between failures) of laptop batteries (measured in months) are shown below:

Table below shows the Time Between Failures

9	11	8	12	6	4	14	11	6	10
16	8	6	7	4	6	7	13	8	16

Assume that the data follows a Poisson process (that is, the time between failures follow an exponential distribution).

Question 7.1 If the manufacturer provides 12 months warranty, what is the expected number of failures during the warranty period?

Question 7.2 The manufacture would like to ensure they can meet the demand for batteries 90% of the times from the stock. How many batteries they should stock to meet this condition for 2-year period?