Answer
It will take Brad 11.1 hours to lose 0.50 kg of fat.
Work Step by Step
We can find the change in gravitational potential energy when the barbell is lifted to a height of $2.0~m$:
$U_g = mgh = (50.0~kg)(9.80~m/s^2)(2.0~m) = 980~J$
Each lift of the barbell requires $980~J$ of work. Therefore, Brad does $2940~J$ of work each minute since he lifts the barbell three times each minute.
This work is only 10% of the energy that Brad gets from burning fat. Therefore, Brad's body uses $29,400~J$ of energy each minute.
We can find the total energy from burning $0.50~kg$ of fat:
$(500~grams)(39,000~J/gram) = 1.95\times 10^7~J$
We can find the time required to use this much energy:
$t = \frac{1.95\times 10^7~J}{29,400~J/min} = 663.3~minutes = 11.1~hours$
It will take Brad 11.1 hours to lose 0.50 kg of fat.
