On Monday, Archie paid $3.40 for three doughnuts and two coffees. On Tuesday he paid $3.60 for two doughnuts and three coffees. On Wednesday he was tired of paying the tab and went out for coffee by himself. What was his bill for doughnut and one coffee?
3D + 2C = 3.40
2D + 3C = 3.60
Multiply top equation by 3 and bottom by 2.
9D + 6C = 10.20
4D + 6C = 7.20
Subtract first equation from second.
5D = 3.00
You should be able to take it from here.