Answer: 15 miles
=======================================================
Here's one approach:
(6 miles)/(24 minutes) = (x miles)/(60 minutes)
6/24 = x/60
6*60 = 24*x
360 = 24x
24x = 360
x = 360/24
x = 15
She travels 15 miles in 60 minutes (aka 1 hour). So we can say her speed is 15 mph.
-----------------------------
Here's another approach:
If she travels 6 miles in 24 minutes, then her unit rate is distance/time = 6/24 = 1/4 = 0.25 miles per minute.
So if she travels for 60 minutes, then she'll cover a distance of 60*0.25 = 15 miles