If a tourist travels the distance between cities A and B on a bicycle, he will spend 2 hours and 30 minutes

If a tourist travels the distance between cities A and B on a bicycle, he will spend 2 hours and 30 minutes less than walking this path. What is the distance between cities A and B if a tourist rides a bicycle at a speed of 12 km / h and walks at a speed of 4 km / h.

Let’s designate the distance between cities as X km.
If a tourist rides a bike, he will spend X / 12 hours on the trip;
And if you walk, it will spend X / 4 hours;
The difference in the time spent – 2 hours 30 minutes = 2.5 hours;
Let’s make the equation:
X / 4 – X / 12 = 2.5h
2 * X = 30
X = 15km.