Solve the Problem $d ( x ) = \frac { D x } { x + 12 } , 0 \leq x \leq 12$

Solve the problem.
-To calculate the drug dosage for a child, a pharmacist may use the formula $d ( x ) = \frac { D x } { x + 12 } , 0 \leq x \leq 12$ .
The child's age is $x$ and the adult dosage is D. What is the dosage for an 4 -year old child if the adult dosage is $90 \mathrm { mg }$ ? (Round to the nearest tenth.)

A) $5.6 \mathrm { mg }$
B) $22.5 \mathrm { mg }$
C) $30.0 \mathrm { mg }$
D) $90.0 \mathrm { mg }$