Why ${\sum=?iso-8859-1?Q?=5F1^n_{s=5Fi}_\over_\sum=5F1^n_{t=5Fi}}_=3D_{n_\over_\sum?=_1^n{t=?iso-8859-1?Q?=5Fi_\over_s=5Fi}}$=3F_?=(harmonic mean) – math.stackexchange.com

Why ${\sum_1^n {s_i} \over \sum_1^n {t_i}} = {n \over \sum_1^n{t_i \over
s_i}}$? (harmonic mean) – math.stackexchange.com

I realized it reading about computer performance. This identity is
presented there with words: dividing total distance by total time, you
arrive at average execution rate on the left. On the right, …