19. Cowling’s rule is a method of calculating a medication dosage for a child by using the child’s age and the adult dosage. The rule uses the formula ， where d is the child’s dosage in milligrams, a is the adult dosage in milligrams, and t is the child’s age in years. Dato is taking a 150-milligram dosage of medication that has an adult dosage of 300 milligrams. According to this rule, which of the following values is closest to Dato’s age in years?