Hi, my first post, and i need help with some applications of calculus to physics

A tank contains 100 litres of brine whose concentration is 3 grams/litres. Three litres of brine whose concentration is 2 grams/litre flow into the tank each minute and at the same time 3 litres of mixture flow out each minute.

Show that the quantity of salt, Q gram, in the tank at any time t is given by:

Q = 200 + 100e^{-0.03t}

i have managed to get Q = 200 - 100e^{-0.03t}

and cannot figure out why the minus is there, i have included the pdf of the working i have done so far, any help appreciated

thanks

jacs