Question 504284
let her ride x miles @ 15mph
then she will ride 30-x @ 6mph
...

Time at 15 mph + time at 6 mph = total time
t=d/r
x/15 + (30-x)/6 = 2.5

LCD =30, Multiply by LCD

2x+5(30-x)=2.5*30
2x+150-5x=75
-3x=-75
x=25 

25 miles @15 mph
5 miles @6 mph