This is a proportion problem, setting up the proportion is the hard part, but once you get that, the calculations are easy.
So, lets set one up
(Let x = the weight of the patient)
[tex]\frac{104lbs}{156mg} = \frac{x}{207mg}[/tex]
Now, Just cross multiply and solve.
104*207 = 156 *x
21528 = 156x
138 = x
The patient requiring 207 miligrams of medicine weighs 138 lbs.
