Since there are 60 seconds in 1 minute, then there are 60 / 30 = 2 sets of 30 seconds in a minute. So a wheel makes 6 x 2 = 12 turns in 1 minute. Since there are 60 minutes in 1 hour, then a wheel makes 12 x 60 = 720 turns in an hour. Therefore, the wheel makes 720 x 2 = 1440 turns in 2 hours.
This justifies what answer for what question? Q & A:
A wheel on a certain machine makes 6 turns every 30 seconds. How many turns does it make in two hours?
1440