For this problem, you know that the first walker will arrive 2 hours before the second, and increases his speed by 2 times the second walker. You also know there is a distance of 24 km. So up until some time x, the two walkers have to be going the same speed. If the first walker increases speed by two times the speed per hour, and arrives two hours earlier, then his initial speed will be 20 km/h, because after 2 hours, he will have an increase of 4 km/hr, and the second will have an increase of 2 km/h, thereby making the first arrive 2 hours earlier, if that makes sense.
