I do not understand where I mistake.

The excercice says:

a philosophy course was taken by 250 students. Each memeber of a random sample of 50 students was asked to estimate the amount of time he spent on the previous week assignment. Suppose that the population standard deviation is 30 minutes.

A) What is the probability that the sample mean exceed the population mean by more than 2.5 minutes?

B) what is the probability that the sample mean is by more than 5 minutes below the population mean?

C) what is the probability that the sample mean differs from the population mean by more than 10 minutes?

Okay. For A I do P(s> p+ 2.5) so P(Z> (p+2.5-p)/(30/Squareroot50) ) look at the table and get 0.2776 but the book says 0.2546.

For B I get 0.121 but the book says 0.0951.

Where am I wrong? :( Thank you for your help.