a) Multiply the usage by the given rate. Add in the fixed charge.
b) Multiply the first 400 units by the given rate. Add in the fixed charge. Multiply the remaining units by the additional rate. Add to the previous amount.
c) Examine your steps in parts (a) and (b), and try to find the pattern. (Do some more examples of unit counts above 400, if necessary.) Then create a formula, using this pattern.