The cholesterol values for a certain population are approximately bell-shaped with a mean of 200 /100mg/100ml and a standard deviation of 20 /100mg/100ml.

a) Determine the proportion that an individual picked at random from this population will have a cholesterol level between 200 and 240 /100mg/100ml.

b) Find the interval, centered on the mean, which contains 68% of all cholesterol levels of this population.