4.15 use the relationship between power, work done (energy transferred) and time taken:

Power = work done / time taken

P = W / t

For example...

A motor transfers 4.8kJ of useful energy in 2 minutes. Find it's power output...

As work done is the same as energy transferred, we know that 4.8kJ of work was done. Therefore...

4800 / (2 x 60) = 4.8 / 120 = 40 W

NOTE: remember to convert kJ to J and minutes into seconds.