Because Marry Kelly are facing very hard decision, she uses Solver in Excel spreadsheet.
First, she makes the decision variables and the model of the problem. In the case, there are 19 decision variables :
Xi = buy new sheets where i is the week
Yi = Wash at the Local Laundry where i is the week
Zi = Washing by Marry’s friends where i is the week
The cost of buying new sheet is $10, wash at the Local Laundry is $4 and by Marry’s friends is $2, and then we can make the model
Minimize C (cost) = 10 ( X1+X2+X3+X4+X5+X6+X7+X8) + 4 (Y1+Y2+Y3+Y4+Y5+Y6) + 2 (Z1+Z2+Z3+Z4+Z5)
Subject to : Week 1 : X1 = 115 Y1 + Z1 = 115 Week 2 : X2 = 210 Y2 + Z2 = 210 Week 3 : X3 + 0.8 Y1 = 250 Y3 + Z3 = 250 Week 4 : X4 + 0.8 Y2 + 0.8 Z1 = 230 Y4 + Z4 = 230 Week 5 : X5 + 0.8 Y3 + 0.8 Z2 = 260 Y5 +Z5 = 260 Week 6 : X6 + 0.8 Y4 + 0.8 Z3 = 300 Y6 = 300 Week 7 : X7 + 0.8 Y5 + 0.8 Z4 = 250 Week 8 : X8 + 0.8 Y6 + 0.8 Z5 = 190 X1, X2,…, Z5 >=0 and integer
For the laundry, it needs a week to finish the washing but Marry deliver the dirty in the weekend so that the clean sheets from week 1 can be used in the week 3. And washing in Marry’s friend, they need 2 weeks to deliver the clean sheets back and the clean sheets can be used in the week 4. 20 % from the washing ( laundry and Marrys friends ) are removed. In week 6, we don’t need to wash at Marry’s friends and in the Week 7 & 8 we don’t need to wash again because it exceeds the sport