Can someone help me correct this:

Question

The formula below gives a rule for the amount of medicine a child should have if you know the age of the child(a) and the amount , or dosage,(d) of the medine an adult would take.Find the child's dosage(d) for the given values of a and D. Round answers to the nearest tenth.

Using formula:
d=a/a+12(D)

when a=5ys,D=2.5ml

My answer:
d=a/a+12(D)
d=5/5+12(2.5)
d=5/17(2.5)
d=.30(2.5)
d=.75 when rounded it would be .80

Using formula:
d=a/a+12(D)

when a=5ys,D=2.5ml

My answer:
d=a/a+12(D)
d=5/5+12(2.5)
d=5/17(2.5)
d=.30(2.5)
d=.75 when rounded it would be .80
I have trouble knowing what the formula is. Is it
d = (a/a) + (12D) or

d = a/(a + 12D). Surely not the first because that leads to a larger number no matter what D is. I will assume it is the latter.
d = a/(a+12D)=
5/(5+12*2.5)=
5/(5+30)=
5/35=1/7 = 0.14 which is 0.1 rounded to the nearest 0.1.
If this is not the correct formula, as I have chosen to use above, please repost and clarify the question.