Solve the Problem $\mathrm { d } ( \mathrm { x } ) = \frac { \mathrm { Dx } } { \mathrm { x } + 10 } , 0 \leq \mathrm { x } \leq 12$

Solve the problem.
-To calculate the drug dosage for a child, a pharmacist may use the formula $\mathrm { d } ( \mathrm { x } ) = \frac { \mathrm { Dx } } { \mathrm { x } + 10 } , 0 \leq \mathrm { x } \leq 12$ . The child's age is $x$ and the adult dosage is D. What is the dosage for an 3 -year old child if the adult dosage is 40 mg? (Round to the nearest tenth.)

A) $3.1 \mathrm { mg }$
B) $12.0 \mathrm { mg }$
C) $9.2 \mathrm { mg }$
D) $40.0 \mathrm { mg }$