The problem is as follows:
An electrical motor of $80\%$ of power efficiency requires $6\,kW$ to propel a centrifuge pump of $73.5\%$ of efficiency, which the latter use it to raise the water from the basement to a water tank situated in the rooftop of the building at a rate of $0.54 \frac{m^3}{min}$. Find the number of floors that the building has if each of them has $2.5\,m$ of height. You may use $g=9.8\,\frac{m}{s^2}$.
The alternatives given in my book are as follows:
$\begin{array}{ll} 1.&14\,floors\\ 2.&16\,floors\\ 3.&12\,floors\\ 4.&10\,floors\\ 5.&18\,floors\\ \end{array}$
I'm still stuck on this problem as I don't know how should I use the information of the efficiency. The only thing which comes to my mind is this formula:
$\eta=\frac{P_{out}}{P_{in}}$
I'm assuming that "out" is the power which is produced by the process and "in" the power put in the process. But I'm confused on where should I put the percentages given and what should I do with the speed in cubic meters per minute. Can somebody help me?. Supposedly the answer is the second option. But I don't know how to get there.