6. The total cholesterol level (LDL + HDL + 20% of triglyceride) of US males between 60 and 70 years of age is approximately normally distributed with a mean of 175 mg/100 ml and a standard deviation of 20 mg/100 ml. Find the probability that a randomly selected person from this population has total cholesterol level: (a)

between 150 and 250 mg/100 ml,

(b) more than 155 mg/100 ml, and

(c) less than 215 mg/100 ml.