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Many of these questions are easily solvable by paying attention to units and thinking for a bit about how to arrive at what is asked, from what is given at a unit level.

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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"
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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.
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"...for which the fuel cost per BTU of usable heat energy produced by this furnace would be approximately the same for each fuel burned alone"

This translates to: CostNG / BTUNG = Cost / BTUP
CostNG / CostP = BTUNG / BTUP = 1000/2500 = 2/5

Move to options: 2/5 ratio exists for 0.0070 & 0.0175
CostNG = 0.0070
CostP = 0.0175
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I got this answer wrong because I flipped the value. The reason I had the value flipped is because I multiplied C x CPU (0.0175 * 1000 = 0.007*2500)
I see in the video we actually divide the C/BTU. I am wondering why we divide?
"fuel cost per BTU of usable heat energy" seems to me to mean the C. I don't see why we would divide fuel cost per BTU by BTU again
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amitarya
Sajjad1994 Bunuel... pls help for the solution..

amitarya I made a video solution for you here:



The key is that our calculation is much less complicated than it might appear at first — the efficiency is the same for both natural gas and propane, so we don't need to take that into account. The only numbers we need are in the 1st sentence.

Therefore, since the cost per BTU must be the same for both, the costs must be in the same ratio as the BTU numbers in the 1st sentence.

So, using the answer choices, we guess and check to find numbers that are in a 1000:2500 (1 : 2.5) ratio:

70*2.5 = 175, so our answers are B and D.
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When they ask for the “fuel cost per BTU of usable heat energy,” they literally mean:

cost in dollars per cubic foot of fuel/usable BTU of heat per cubic foot of fuel

Since you’re dividing by the BTU (not multiplying), you convert from “\(\frac{$}{ft^3}\)” into “\(\frac{$}{BTU}\)”

lily123400
I got this answer wrong because I flipped the value. The reason I had the value flipped is because I multiplied C x CPU (0.0175 * 1000 = 0.007*2500)
I see in the video we actually divide the C/BTU. I am wondering why we divide?
"fuel cost per BTU of usable heat energy" seems to me to mean the C. I don't see why we would divide fuel cost per BTU by BTU again
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amitarya
Sajjad1994 Bunuel... pls help for the solution..

amitarya I made a video solution for you here:



The key is that our calculation is much less complicated than it might appear at first — the efficiency is the same for both natural gas and propane, so we don't need to take that into account. The only numbers we need are in the 1st sentence.

Therefore, since the cost per BTU must be the same for both, the costs must be in the same ratio as the BTU numbers in the 1st sentence.

So, using the answer choices, we guess and check to find numbers that are in a 1000:2500 (1 : 2.5) ratio:

70*2.5 = 175, so our answers are B and D.
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