The question above isn't entirely clear as far as "Bob" and "her." If Bob made the journey in less time a slower rate, it doesn't work. Unless Bob is female (the "her").
If Person1 makes a journey of 3 hrs at one speed and Person2 makes the same journey in 4 hrs at another speed (10 km/hr slower), how long is the journey?
Person1 distance traveled: 3 hrs * X km/hr
Person2 distance traveled: 4 hrs * (X-10) km/hr
They made the same journey, so they traveled the same distance, so we can do this:
3 * X = 4 * (X - 10)
3X = 4X - 40
3X + 40 = 4X
40 = X
Person1 distance traveled: 3 hrs * 40 km/hr = 120 km
Person2 distance traveled: 4 hrs * (40-10) km/hr = 4hr * 30 km/hr = 120 km
The journey was 120 km long.