Question

Polluted water flows into a pond. The concentration of pollutant, c, in the pond at time, t, minutes is modeled by the equation c(t) = 9 - 90 000 (1/ 10 000 + 3t)

a) When will the concentration of pollutant in the pound reach 6kg/m^3?

Answer 1

I suspect you meant:
c(t) = 9 - 90 000 (1/ (10 000 + 3t) )
I will also assume that c(t) is in kg/m^3

then
6 = 9 - 90 000 (1/ (10 000 + 3t))
90000(1/(10000+3t)) = 3
30000 + 9t = 90000
9t = 60000
t = 6666 2/3 minutes ?

check my arithmetic