Question

If you want to lose 4 lb of "body fat," which is 15% water, how many kilocalories do you need to expend?

Answer 1

I don't know how advance this class is nor how detailed this is to be done but I will do it the easy way.
I understand that it takes 3500 kcal to be expended for every lb of "body fat" lost; therefore, 4 lb x 3500 kcal/lb = about 14,000 kcal.
You realize that 3500 kcal/lb is an average. You may have used a different value in your classes so use that number if you have it at hand. Good luck.

Answer 2

You may be interest in knowing that I found a way to estimate that 3500 number. So-called "pure" fat is 9 kcal/g and since this body fat in the problem is listed as 85% fat (15% water) then that 9 kcal/g will be
9 kcal/g x 0.85 = 7.65 kcal/g for this 85% stuff. Then
7.65/g x 454 g/lb x 4 lb = 13,892 kcal to be expended which is quite close to that 14,000 estimated above. That 14,000, by the way, was based on close to 86% water.

Answer 3

4lbs = 15% water, 85% fat
4 lbs = 1814.37 g

1,814.37 g x 0.85 = 1,542.2145 g

Assuming fat provides energy at 9 kcal/g,
1,542.2145 g x 9 kcal/g = 13,879.9305 kcal

Rounding to 1 sig fig: 10,000 kcal