Concentration in brine solution

[jacs]

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

[jacs]

i cant seem to upload my attachement for abvoe post, going to try it here


grrr can't make it work here either is there something special needing to be done to make the attachement work aside from uploading it?

[jacs]

i figure it out and i dont appear to be able to delete the post, which is no longer necessary.