Old hens can be bought for Birr 2 each but young ones costs Birr 5 each. The old hens lay 3 eggs per week and the young ones lay 5 eggs per week. Each egg costs Birr 0.30. A hen costs Birr 1 per week to feed. If the financial constraint is to spend Birr 80 per week for hens and the capacity constraint is that total number of hens cannot exceed 20 hens and the objective is to earn a profit more than Birr 6 per week, find the optimal combination of hens.