#### Answer

t = 4.6 x $10^{4}$ h = 5.25 yr $\approx$ 5.3 yr

#### Work Step by Step

Rearranging Equation 17.23 to get time t,
t = $\frac{KW}{pA(CPR)}$
Using the given values and substituting it to the above equation:
Use CPR value of 87.6:
t = $\frac{(87.6)(7.6 \times 10^{6} mg)}{(4.5 g/cm^{3})(800 cm^{2})(4 mm/yr)}$
t = 4.6 x $10^{4}$ h = 5.25 yr $\approx$ 5.3 yr