Since I haven't commented . . .

Assuming that an engine turns a constant 7200 RPM for 4.5 seconds, it will make exactly 540 revolutions. (7200/60) x 4.5 = 540. We all know those assumptions are silly for a TFD, but under those assumptions, that math is correct.

As for dividing 7200 by 4.5, that math has no meaning. If you look at the units (dimensional analysis), they are Revs/minute. If you divide that by 4.5 seconds, the final units would revs per minute-second, which doesn't represent anything in particular.