a
For just complete rotation
$v = \sqrt {Rg} $ at top point
The rotational speed of the drum
$ \Rightarrow \omega = \frac{v}{R} = \sqrt {\frac{g}{R}} = \sqrt {\frac{{10}}{{1.25}}} $
The maximum rotational speed of the drum in revolutions per minute
$\omega \left( {rpm} \right) = \frac{{60}}{{2\pi }}\sqrt {\frac{{10}}{{1.25}}} = 27.$