This question requires you to use the formula

Freq = RPM X No of Pole Pairs /60

This formula is used for AC generators and AC motors.

What you must look out for in this question is that you are not given the number of pole pairs, but told there are 4 magnetic poles. This means there are 2 pole pairs (4/2)

400 =RPM X 2 / 60 Transposing the formula for RPM =

400 X 60 / 2 = 12000 RPM