A person travelled a distance of 50 km in 8 hours. He covered a part of the distance on foot at the rate of 4 km per hour and a part on a bicycle at the rate of 10 km per hour. How much distance did he travel on foot?

To find the distance travelled on foot, we can use the method of substitution or an equation. Let the time spent walking be x hours. Since the total time is 8 hours, the time spent cycling must be 8 minus x hours. The total distance is the sum of the distance covered on foot and the distance covered by bicycle. Distance equals speed multiplied by time. So, 4x + 10 multiplied by (8 minus x) equals 50. Expanding the equation: 4x + 80 minus 10x equals 50. Simplifying: 80 minus 6x equals 50. This gives 6x equals 30, which means x equals 5 hours. Since the person walked for 5 hours at a speed of 4 km per hour, the distance travelled on foot is 5 multiplied by 4, which equals 20 km. Alternatively, using the options, if he walked 20 km, it took him 5 hours (20 divided by 4). This leaves 3 hours for cycling. In 3 hours at 10 km per hour, he covers 30 km. The total distance is 20 plus 30, which equals 50 km. This confirms option B is correct.