Cooper Section $11\mathrm{.}0$ #$26$

A lake has a volume of ${10}^8{m}^3.$ A river flows into the lake at the rate of $4{10}^5{m}^3/$ day. The same amount of pure water evaporates from the lake each day, so the volume of the lake does not change. The river contains $7\%$ pollution. The lake is initially unpolluted.

a$.$ What is the percentage pollution in the lake after $t$ days?

The percentage pollution is $\%$

b$.$ When does the pollution in the lake reach $1\%?$

After days