Jessy runs 4 miles in 30 minutes. At this rate, how far could she run in 45 minutes?  

Let's assume the unknown quantity here is x. 

In this case, x is the number of miles Jessy can run in 45 minutes at the given rate. 

It is given that running 4 miles in 30 minutes is the same as running x miles in 45 minutes. 

The equation of proportion for the above case can be written as:

4 : 30 :: x : 45

$\frac{4}{30} = \frac{\mathrm{x}}{45}$

30 × x = 4 × 45

x = $\frac{180}{30}$ = 6. 

Therefore, Jessy can run 6 miles in 45 minutes