Question: Monthly Utility bills are normally distributed with a mean value of $160 and standard deviation of $25.
a. Find probability of bill between 120 and 180
b.Find probability of bill less than 120
c.Find probability of bill more than 210
  Edit

Answer: Define X as the monthly utility bill and Z as a standard normal random variable.
(a) P(120 < X < 180) = P((120-160)/25 < Z < (180-160)/25) = 0.7333
(b) P(X < 120) = P(Z < (120-160)/25) = 0.0548
(c) P(X > 210) = P(Z > (210-160)/25) = 0.0228  Edit