In order to calculate the child's dosage, let's use the value of n = 4 and M = 180 in the formula, then we calculate the value of D:
[tex]\begin{gathered} D=\frac{n}{n+12}\cdot M \\ D=\frac{4}{4+12}\cdot180 \\ D=\frac{4}{16}\cdot180 \\ D=\frac{1}{4}\cdot180 \\ D=\frac{180}{4} \\ D=45 \end{gathered}[/tex]So the child's dosage is 45 milligrams.