normal distribution problem

Suppose that the time spent each month on a mobile-phone by a subscriber with a particular profile is normally distributed with a mean of 175 minutes and a standard
deviation of 45 minutes.
A mobile-phone operator offers the subscriber a monthly flat rate plan:
• 90 minutes’ use or less costs $35
• between 90 minutes’ and 240 minutes’ use costs $60
• more than 240 minutes’ use costs $75.

Question 1:
Assuming that the subscriber’s use of the phone does not change, calculate the mean and standard deviation of the subscriber’s monthly charges.

Question 2:
Why is the assumption that the use will not change unrealistic?