Harmonic mean
Sequences, series and the binomial theorem · Mathematics
Study notes
Q: A car goes 60 km at 30 km/h and returns 60 km at 60 km/h. Average speed? Step 1: Time out = 60/30 = 2 h. Time back = 60/60 = 1 h. Step 2: Total distance 120 km, total time 3 h. Step 3: Average = 120/3 = 40 km/h. Check HM: 2×30×60/90 = 40 ✓. (Not the AM 45!)