Answer
See explanation.
Work Step by Step
a. RBE = 1 so 0.25 mSv corresponds to 0.25 mGy.
Energy = $(0.00025J/kg)(5.0 kg)=1.2 \times10^{-3}J$
b. RBE = 1 so 0.10mGy=10mrad and the dosage is 10 mrem.
Energy = $(0.00010J/kg)(75 kg)=7.5 \times10^{-3}J$
C. $\frac{7.5 \times10^{-3}J }{1.2 \times10^{-3}J }=6.2$
Each chest x ray delivers roughly a sixth of the yearly energy received from background radiation.
