Working speed in rpm

A transmission shaft is fitted with a coupling that is secured by eight 20 mm diameter bolts pitched on a circle with a diameter of 225 mm. The bolt material has an ultimate shear stress of 280 MN/m2 and a factor of safety of 4.0 is to be used. If the power being transmitted is 1.8 MW, determine the working speed of the shaft in revolutions per minute.

Please show workings and solution.