一、求解下列問題：
(a)A tank contains 1000 liters of water in which initially 100 kilograms
of salt is dissolved. Brine runs in a rate of 10 liters per minute, and
each liter contains 5 kilograms of dissolved salt. The mixture in the
tank is kept uniform by stirring. Brine runs out at 10 liters per minute.
Find the amount of salt in the tank at any time t. (15 分)