Water falls from a 40m high dam at the rate of 9 × 10 ^ 4 kg per...

Since $50$ efficiency is given, half of the energy of the falling water will be used by the bulbs. Therefore,

$\dfrac{1}{2}\dfrac{∆ mgh}{\Deltat} = n \times P ⇒ \dfrac{1}{2} \times \dfrac{9 \times {10}^{4}}{3600} \times 10 \times 40 = 100n ⇒ n = 50$