Here's the information we have:
When fully burned,
natural gas produces approximately 1,000 BTU of heat per cubic foot of fuel
propane produces about 2,500 BTU of heat per cubic foot of fuelSo, the ratio of the amount of heat produced by a cubic foot of natural gas to the amount of heat produced by a cubic foot of propane is 1000/2500 = 1/2.5.
Then the second paragraph says the following:
Kaiser will purchase a furnace whose efficiency with respect to either natural gas burned alone or propane burned alone is 90%.This information does not change the ratio we already have since (0.9 × 1000)/(0.9 × 2500) = 1000/2500 = 1/2.5.
In the table select for natural gas cost per cubic foot and for propane cost per cubic foot the values that are jointly consistent with the information given 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. Make only two selections, one in each column.Since the two fuels produce heat in the ratio of 1/2.5 per unit, to make the fuel cost per unit the same, we can simply find two prices such that the ratio of prices for natural gas and propane is also in the ratio of 1/2.5.
0.0035
0.0070
0.0110
0.0175
0.0350The only two prices that are in the ratio of 1/2.5 are 0.0070 and 0.0175.
For
Natural gas cost per cubic foot, select
0.0070.
For
Propane cost per cubic foot, select
0.0175.