Help me solve a differential equation

Differential Equation

Hi
HELP!!!

Can anyone help me with this one pls? I can't quite figure it out. Thanks in advance.

Problem:

A lake having a storage capacity of 10,000 acre-ft is lowere until it contains only 1000 arce-ft of water, at withc time it is treated with 2 tons of a toxic chemical to rid it of undersirabe aquatic life. New species are to be introduced which can tolerate no mre than 10^-4 lb of chemical per arce-foot of water. Fresh water enters the lake via several streams at a rate of 5 acre-ft/hr, and water is released from the lake at the rate of 4 acre-ft/hr. Assuming the chemical is quickly and uniformly distributed throughout the lake, when will the amount of chemical reach the tolerance level of the new species so that they may be planted? How much more water will the lake hold at that time?

Thanks Again!

This does not require differential equations.
Post this question on a chemical engineering board.

Sounds like a Constant Stirred Tank Reactor problem...first order ODE and yes a chem eng problem.
rt