Water is draining from a tank. The graph
of f(x), the amount of water in the tank,
in gallons, as a linear function of x, the
number of hours water has been draining,
passes through the points (20, 0) and
(0, 80). What is the rate of change in f(x)
with respect to x?

The management of a supermarket wants to adopt a new promotional policy of giving a free gift to every customer who spends more than a certain amount per visit at this supermarket. the expectation of the management is that after this promotional policy is advertised, the expenditures for all customers at this supermarket will be normally distributed with a mean of $95 and a standard deviation of $20. if the management wants to give free gifts to at most 10% of the customers, what should the amount be above which a customer would receive a free gift?