If I am riding my push mountain bike at 25 km/h, and my tyre radius is 30.5 cm.

How many revolutions per minute are my wheels travelling at?

$25 \quad\text{km/h}= \frac{25000 \quad\text{m}}{60 \quad\text{mins}} = 416.\dot{6}\,\,\text{m/min}$
$1$ revolution of the wheel covers $2\pi r= 2\pi(0.305)=1.9163...\text{m}$ so $\416.\dot{6}\,\,\text{m}=\frac{416.\dot{6}}{1.9}\, \,\text{revs}$