The correct formula for the dosage (d) of the medicine for a child, given the age (a) and the adult dosage (D), is:
d = a / (a + 12D)
In this case, when a = 5 years and D = 2.5 ml, we can substitute these values into the formula and solve for d:
d = 5 / (5 + 12 * 2.5)
d = 5 / (5 + 30)
d = 5 / 35
Now we can simplify the expression by dividing the numerator by the denominator:
d = 1 / 7
This fraction can be approximated to the nearest tenth by dividing 1 by 7, which equals approximately 0.142857. Rounded to the nearest tenth, the child's dosage is 0.1 ml.