1. The problem statement, all variables and given/known data The life time of electric lights brand A and B independent and normally distributed .For brand A bulbs the mean and standard deviation of the life time are 1010 hours and 5 hours respectively .For brand B those values are 1020 and 10 hours calculate the probability that a Brand A bulb will have a longer life than brand B 2. Relevant equations 3. The attempt at a solution If this question was asking about a probability of brand A to have a life time of a particular value (as an example 1000 hours ) then i know how to calculate the probability by converting this distribution to a Standard Normal Distribution by using Z score .then You can easily obtain the value by given tables or by a calculator .That can be also done to brand B. But the problem is i dont how to connect these two distribution >please someone help on this question .