A batch of 5000electric lamps has a mean life of 1000 hours and a standard deviation of 75hours.Assume a normal distribution.

(a)How many lamps will fail before 900 hours?

(b)How many lamps will fail 950 and 1000 hours ?

(c) what proportion of lamps will fail before 925 hours?

(d)Given the same mean life,what would the standard deviation have to be ensure that no more than 20percent lamps fail before 916 hours