Calculate the ratio of the concentration of the $22\mathrm{.}7-$sec precursor to the thermal neutron density in a critical infinite thermal reactor consisting of a mixture of ${?}^{235}U$ and ordinary water, given that the thermal flux is ${10}^{13}$ neutrons $\frac{?}{c{m}^2}-$sec$.[$Hint: Place $d\frac{C}{d}t=0$ in Eq$.(7\mathrm{.}21).]$