Quabbin Reservoir in Massachusetts provides much of Boston’s water. At the start of 2009 the reservoir contained 412 billion gallons of water. (a) If in January 2009 the amount (in billions of gallons) flowing into the reservoir is A and the amount flowing out is B , write an expression for the amount of water in the reservoir at the end of January 2009. (b) If in February 2009 the amount flowing in is 20 billion gallons less than the previous month and the amount flowing out is 12 billion gallons more than the previous month, write an expression for the amount of water in the reservoir at the end of February 2009. (c) Combine like terms to simplify your answer to part (b).