Question

The monthly electrical utility bills of all customers for New River Light and Power are known to be distributed normally with a mean equal to $50 a month and a population standard deviation of $10. If a random sample of n=100 customers is taken, what is the probability that the average bill of that sample will exceed $60

Answer 1

Solution :

mean = = 50

standard deviation = = 10

n = 100

= = 50

= / n = 10 / 100 = 1

P( > 60) = 1 - P( < 60)

= 1 - P[( - ) / < (60 - 50) / 1 ]

= 1 - P(z < 10)

Using z table,    

= 1 - 1

= 0