LCM OF 9, 15 and 21 is 315

Change 315 to hours and it will be 5hrs 15 minutes

This means that it will take 5hours 15 minutes intervals

Get the time difference between 10.00am and 5.45pm and it is 16 hours 15 minutes

Then divide the difference with the time interval to get the number of times

And the answer is 3 times.