Water falls from a height of 60m at the rate of 15kg s ^ - 1 to...

Change in potential energy of water per second $= 15 \times g \times 60$

$= 9000J{s}^{- 1}$

Energy remaining per second after loss $= 90$ of $9000J{s}^{- 1}$

$= \left(\dfrac{90}{100}\right) \times 9000$

$= 8100J{s}^{- 1}$

$= 8 . 1kW$