Thomas Young has suggested the following rule for calculating the dosage of medicine for children 1 to 12 yr old. If a denotes the adult dosage (in milligrams) and if t is the child's age (in years), then the child's dosage is given by the following function. at t + 12 Suppose the adult dosage of a substance is 660 mg. Find an expression that gives the rate (in mg/year) of change of a child's dosage with respect to the child's age. D'(t) = D(t) = What is the rate of change (in mg/year) of a child's dosage with respect to his or her age for a 4-yr-old child? A 12-yr-old child? (Round your answers to three decimal places.) 4-yr-old 12-yr-old mg/year mg/year