A utility company offers a lifeline rate to any household whose electricity usage falls below 240 kWh during a particular month. Let A denote the event that a randomly selected household in a certain community does not exceed the lifeline usage dur- ing January, and let B be the analogous event for the month of July (A and B refer to the same household). Suppose P(A) = .8, P(B) = .7, and P(A U B) = .9.

Compute the following:

a. P(A n B).

b. The probability that the lifeline usage amount is exceeded in exactly one of the two months. Describe this event in terms of A and B.