Suppose that the duration of trouble-free operation of a new vacuum cleaner is normally distributed with a mean of 550 days and a standard deviation of 100 days. The research department randomly selects 25 customers who bought these vacuum cleaners and records the duration (in days) of trouble-free operation.Which of the following isNOTTRUEfor the sample average of number of trouble-free operation days (x bar)?

1. P (x< 500) = P(Z < -2.5)
2. P(x> 600) = P(Z > 2.5)
3. P(x< 520) = P(Z < - 1.5)
4. P(550