The characteristic compressive strength of concrete required in a project is $25 \: MPa$ and the standard deviation in the observed compressive strength expected at site is $4 \: MPa$. The average compressive strength of cubes tested at different water-cement (w/c) ratios using the same material as is used for the project is given in the table.
$\begin{array}{|c|c|c|c|c|} \hline w/c (\%) & 45 & 50 & 55 & 60 \\ \hline \text{Average compressive strength of cubes (MPa)} & 35 & 25 & 20 & 15 \\ \hline \end{array}$
The water-cement ratio (in percent, round off to the lower integer) to be used in the mix is ______