Correct Answer - Option 1 : 81 RPM

__Concept:__

Specific speed of a centrifugal pump is given by:

\({N_s} = \frac{{N\sqrt Q }}{{H_m^{3/4}}}\)

__Calculation:__

**Given:**

Q = 810 l/s = 0.81 m^{3}/s, H = 16 m, N = 725 RPM

\({N_s} = \frac{{N\sqrt Q }}{{H_m^{3/4}}}\) = \(\frac{{725\sqrt{0.81}}}{{16^{3/4}}}\) =** ****81.56 RPM**

**Option a is closed to the answer.**