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