Try calculating the derivative at the point when 50% remains (Hint when 50% remains, then x = 0.5).
Instantaneous rate of change help!!?
Find and interpret the instantaneous rate of change when 50% remains?
D(x) = 1.66 - 0.90x + 0.47x^2
X is the fraction of remaining brine when x is greater than or equal to 0 or less than or equal to 1