Refer to page 106 of the FE Handbook: Pump Power Equation: Top Left on Page.

Power (ft lb/s) = Q ɣ h / n

Givens:

Q = 5 ft3/s

h = 25 ft

n = .72

ɣ = 62.4 lb/ft3

(Note: if ɣ was not given in the problem, simply look it up on the table on page 110 Properties of Water) If they gave you the temperature of the water, use it to find the specific weight for that temperature)

Therefore:

Power (ft lb/s) = (5 ft3/s 62.4 lb/ft3 25 ft) / .72

Power (ft lb/s) = 10,833.33 ft lb/s

However, be careful! The equation solved for the power in ft lb/s, not horsepower.

Lets convert to horsepower:

Refer to page 2 of the FE Handbook: Conversion Factors:

10,833.33 ft lb/s (1.818 Χ 103)

= 19.69 horsepower (therefore, select the 20 horsepower pump)

The answer is (D) 20 horsepower.