A man covers a distance of 20 km in 3.5 hours partly by running and partly by walking. If he walks at 4 kmph and runs at 10 kmph, find the distance he covers by walking.

A. 10 km
B. 8 km
C. 12 km
D. 9 km

Let the distance covered by walking be x km
the distance covered by running be (20-x) km Time for walking + time for running = 3.5 hours (x/4)+[(20-x)/10]=3.5 Solving,
5x+40-2x = 70
3x=30
x=10km