A lake has a volume of 10^8 m^3. A river flows into the lake at the rate of 3×10^5 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? __________%
(b) When does the pollution in the lake reach 1%? after ______ days