Linear Programming Minimization Problem

P. A hotel rental service needs to have clean towels for each day of a three-day period. Some of the clean towels may be purchased new and some may be dirty towels from previous days that have been washed by a laundry service. The cost of new towels is $1 per towel, cost of a fast one day laundry service is 40 cents per towel and the cost of a slow two day laundry service is 25 cents per towel. If the rental service needs 300,200, and 400 clean towels for each of the next three days, respectively. how many towels should the rental service buy new and how many should rental service have washed by different laundry services so as to minimize total costs?

Now, clearly this is a minimization problem and I can solve for it using a minimization tableau. But I don't really know how to set up the problem. The back of the book says there are 4 variables to solve for, but I only count 3, so I have no idea how to proceed. Please shed some light on this problem. Thanks in advance.