Or, more simply, (50/60)^3AFAIK, when your speed changes in a fan/pump, the afffinity laws are:
1) Volume output/discharge: Q1/Q2 = N1/N2, in this case : Q2 =5/6 x Q1 = 83%Q1 (17% decrease)
2) Change in head/pressure: p1/p2 = (N1/N2)^2: p1 = 25/36 x p1 = 69%p1 (31% decrease)
Lastly:
3) Power drawn: kW1/kW2 = (N1/N2)^3; kW2 = 58%kW1 (42% decrease)