Remark
What the question is saying is that you are setting up a proportion. Ordinarily a child would require 125 mg be put into 5 mL of liquid. This child is special and only requires 100 mg. How many mL are needed?
Proportion
125 mg : 5 mL :: 100 mg: x
Solution
125 / 5 = 100 / x Cross multiply
125x = 500 Divide by 125
125x/125 = 500/125 Do the division
x = 4
Answer
100 mg would be put in 4 mL of liquid