If a man swims 4 miters upstream at 1 mph and back downstream to the same point at 4 mph, what is his average speed?

Description (বিবরণ) :

প্রশ্ন: If a man swims 4 miters upstream at 1 mph and back downstream to the same point at 4 mph, what is his average speed?

ব্যাখ্যা: Let towards upstream he swims at v1 mph and towards downstream he swims at v2 mph . '. v1 = 1 and v2 = 4 Now, his average speed $$\begin{gathered} =\frac{2}{{\displaystyle \frac{1}{v_1}+\frac{1}{v_2}}}=\frac{2}{{\displaystyle \frac{1}{1}+\frac{1}{4}}} \\ =\frac{2}{{\displaystyle 1+\frac{1}{4}}}=\frac{2}{{\displaystyle \frac{5}{4}}}=\frac{2}{{\displaystyle \frac{5}{4}}}=2\times \frac{4}{3}=\frac{8}{5}=1.6 mph \\ [Rule of harmonic mean ] \\ Total 5 mph swims 8 mitres \\ 5\left|8\right| 1.6 mph \\ \frac{ 5}{{\displaystyle \frac{30}{{\displaystyle \frac{30}{\times }}}}} \end{gathered}$$