MercedesF1
KarishmaB GMATCoachBen - If the energy produced is in the ratio of 2:5 then shouldn't price be in the ratio of 5:2 as the question stem mentions " BTU of usable heat energy produced by this furnace would be approximately the same for
each fuel burned alone"
BTUs of energy produced is in the ratio 2 : 5 BTUs per cubic foot.
BTUs of usable heat energy produced will be in the ratio 0.9*2 : 0.9*5 (Efficiency of the furnace is 90% for both fuels)
This ratio is still 2:5. Say natural gas produces 2 BTU of usable energy per cubic foot and propane produces 5 BTU of usable energy per cubic foot
Usable heat energy is based on the efficiency of the burner. It has nothing to do with pricing. When heat is produced how much of it can be used effectively and how much dissipates in the furnace - that decides the efficiency of the furnace. Here 90% of the heat is used. Hence usable energy is simply 90% of energy produced.
If FUEL COST per BTU of usable energy should be same, fuel cost should be in the ratio 2:5.
One possibility is fuel cost for natural gas could be $2 (for 2 BTU of usable energy) per cubic foot and for propane $5 (for 5 BTU of usable energy) per cubic foot.
In this case cost per BTU of usable energy for both fuels is the same $1/BTU of usable energy.