Researchers who have been studying the alarming rate at which the level of the Dead Sea has been dropping have shown that the density d(x) (in g per cm³) of the Dead Sea brine during evaporation can be estimated by the function d(x) = 1.66 - 0.90x + 0.47x², where x is the fraction of the remaining brine, 0 ≤ x ≤ 1.

(a) Estimate the density of the brine when 50% of the brine remains.

(b) Find and interpret the instantaneous rate of change of the density when 50% of the brine remains.