Tannish travels from A to B at a speed of 20 km/h, from B to C at 15 km/h and from C to D at 30 km/h. If AB = BC = CD, then find Tannish’s average speed for the whole journey.
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e 20 km/h.
Let the distance of each segment (AB, BC, CD) be x km
Total distance = x + x + x = 3x
Total time = AB + BC + CD = x/20 + x/15 + x/30
LCM of 20, 15, 30 = 60
x/20 = 3x/60; x/15 = 4x/60 and x/30 = 2x/60
Total time = (3x + 4x + 2x) / 60 = 9x / 60
Average speed = total distance ÷ total time = 3x ÷ (9x / 60) = 3x × (60 / 9x) = (180x) / 9x = 20 km/h