#### 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.