The General idea is this...

where A is the amount in the tank

is the rate in and is the rate out

Since the tank is 200L and the concentration is 1g/L we have 200g at time t=0.

The rate of die comming in is zero.(They are only adding pure water)

So we get the ODE

with intial condition

This is first order linear and seperable. So solving we get

Good luck.

TES