We have seen that Young's rule
$$
C=\frac{D A}{A+12}
$$
can be used to approximate the dosage of a drug prescribed for children. In this formula, $A=$ the child's age, in years, $D=$an adult dosage, and $C=$the proper child's dosage. Use
this formula to solve Exercises $73-74$
When the adult dosage is 1000 milligrams, a child is given 500 milligrams. What is that child's age?