Researchers have observed that the mass of a female (gilt) pig can be estimated by the function M(t) = -3.5 + 197.5e^{-e-0.01394(t - 108.4)}, where t is the age of the pig (in days) and M(t) is the mass of the pig (in kilograms).

(a) If a particular gilt is 80 days old, use differentials to estimate how much it will gain before it is 90 days old.
(b) What is the actual gain in mass?