There's a catch!
The simple answer is to divide 550 by 60 to get 9.1(6) RPS, but it depends on how the 550 figure is obtained. The obvious way is to run a stopwatch for one minute and count how many rounds were fired.
However, if the bullets are counted with B~1 fired at time t=0 sec, and B~550 fired at time t=60 sec, then only 549 bullets are fired "within" the minute. That is, the first bullet is fired pretty much instantaneously, but the remainder have a lag time for reloading. If your stopwatch test counted exactly 550 rounds in one minute, a test of two minutes would only count 1099 bullets.
As for rounds-per-second:
With bullet B~1 fired at t=0, B~2 is fired at t=0.1092896175… sec (based on [the remaining] 549 per minute), This means 9.15 bullets are fired in the first second AFTER the very first bullet. Thus in any given second, it is not inaccurate to say the M60 fires 10.15 bullets. BUT, multiplying 10.15 by 60 does not give you the rated 550 RPM, just as 2 × 550 does not give you the real count of rounds fired in two minutes. There's always a catch.