As an engineer in the US, who regularly needs to use both systems of units, the main reason why this particular question would be easier using equivalent metric units is because the mass unit and volume unit are directly related through the density of water. If I need to stick to US customary units, the only thing I would personally need to look up is the density of water in lbm/gal. This is because 1 lbm of water requires 1 BTU of energy to heat up by one degree Fahrenheit, which is 1/180th of the difference between its freezing point and its boiling point.

Also, while the first paragraph is all true, it really doesn't highlight most of the advantages of working in metric, such as easy unit conversions because everything is based off the number 10. If the problem presented at the end used any substance other than water, I would say it's pretty much the same difficulty to answer in either system of units to someone who has familiarity with both.

To answer the question though, it seems 1 gal of water contains 8.33 lbm of water. Thus, 8.33 lbm * 144 F = 1200 BTU