I'm having a bit of trouble with some questions, I'm not sure whether it is from deficiancies in D.E or integration and would like some help if possible.

so here's an example.

A tank with 200L of pure water initially, has 4grams of salt per litre run into the tank at 1L per min. A well mixed solution is drained out at 3L per min.

I am to work out the amount of salt in the tank after t mins. Here's what im doing:

V=200-2t--------volume function of time

Dx/Dt=4-(3x/200-2t)------transpose

Dx/dt+(3x/200-2t)=4

integrating factor= e^((integral of)3/200-2t)

=(200-2t)^(-3/2)

Equation then becomes

x(200-2t)^(-3/2)={integral of}4((200-2t)^(-3/2))

Transposing

x =4(200-2t)^(-.5)+c

This equation doesn't seem to be realistic as shouldn't the amount of salt in the container look like a parabola or trig(sin/cos) function?

Thanks in advance