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Concentration in brine solution

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