15-8=7 (hours)

Now it needs 7-4=3 (hours) * * * Need: 8+3= 1 1 (hours).

Suppose the original speed is x kilometers per hour, and the current speed is y kilometers per hour.

2x+50=y

7x=3y

Solution: x= 150 y=350.

Total distance: 150* 15

Average hourly speed; 150 *15/1=10 km/h