A certain musician is given in an amount proportional to a patients body weight. Suppose a patient weighing 119 pounds requires 136 milligrams of medicine. What is the weight of the patient who requires 200 milligrams of medicine

You would use fractions in my opinion.
0.25 is easy to change to a fraction. It's 1/4
However, 1/3 is not easy to change to a decimal because it would be the repeating decimal 0.333333(going on forever).

So basically it's just easier to change them both to fractions because 1/3 isn't a terminating decimal.