$20\mathrm{.}9$ An adjustment of boron content from $1500$ to $1400$ ppm is made in the reactor described in Excersice $20\mathrm{.}8.$ Pure water is pumped in$,$ and then mixed coolant and poison are pumped out in two separate steps. How long should the $500$ ft$3/$min pump operate in each of the operations? $($Answer is $1\mathrm{.}43$ minutes$)$