Answer
900 chest x-rays.
Work Step by Step
RBE = 1.0 for x rays, so the equivalent dose in Sv equals the absorbed dose in Gy, or J/kg.
One whole-body scan delivers an energy of $\frac{0.012J}{kg}(75kg)=0.90J$.
One chest x ray delivers an energy of $\frac{0.0002J}{kg}(5.0kg)=1.0mJ$
The ratio $\frac{0.90J}{0.0010J}$ is 900. It would take 900 chest x rays to deliver the same total energy as a whole-body CT scan.
