Question 168099
I'm not sure how they wrote the formula - but when I looked it up it said "fraction of the adult dose obtained by dividing the age of the child at the nearest birthday by 24"

as the adult dose is 600 and the child dose is 200 and we have 

d=D*((a+1)/24) I think you were saying 

d/D will = age + 1 divided by 24

so 200/600 = 1/3      

so now we have 1/3 on one side  and (age +1)/24 on the other side

multiply both sides by 24 to give 24/3 = age = 8 I think