Butne has four carbons, so each liter of butane will generate four liters of carbon dioxide. Propane has three carbons, so each liter of propane will generate three liters of carbon dioxide. Let x = the number of liters of butane in the 3 liters of butane/propane mixture. Then 3-x would be the number of liters of propane in the mixture.
There are 10 liters of carbon dioxide generated, so
10 = 4x + 3*(3-x)
10 = 4x + 9 - 3x
10 = x + 9
1=x, so there is 1 liter of butane, therefore there are 2 liters of propane. The ratio of butane to propane is
therefore 1 to 2.