If 500 thousand btus of energy are available to raise the temperature of a water boiler from 20â°f to 100â°f, how many gallons of water can be added to the boiler?

In this question, you are given the energy( 500,000btus) and the temperature difference( 100F-20F= 80F). You are asked to find the volume of water.
1 BTU mean 1 degree of F increased for 1 pound mass. First, we can calculate the mass of the water in lb unit. The calculation would be:

mass= energy/temperature increase = 500,000/80= 6250lb.

Then we need to convert the weight into gallons. The calculation would be:
6250lb x kg/2.204lb x 1kg/m3 x 264.172 gallons/m3= 3638969.3 gallons

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Independent variable is something that is changed in a experiment.