A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is

To find the average speed when equal distances are covered at different speeds, we use the formula for harmonic mean. Let the total distance be divided into three equal parts of D each. The formula for average speed is Total Distance divided by Total Time. Total Distance equals D plus D plus D which is 3D. Time taken for the first part is D divided by 10. Time taken for the second part is D divided by 20. Time taken for the third part is D divided by 60. Total Time equals D divided by 10 plus D divided by 20 plus D divided by 60. Taking 60 as the common denominator, the total time is 6D plus 3D plus 1D all divided by 60, which simplifies to 10D divided by 60 or D divided by 6. Average Speed equals 3D divided by the quantity D divided by 6. This simplifies to 3 multiplied by 6, which equals 18 km/hr. Therefore, option A is the correct answer.