At a telethon, a volunteer can take 48 calls over a 4-hour shift. at this rate, how many calls can 12 volunteers take in a 4-hour shift?

1 volunteer - 4 h - 48 calls
12 volunteers - 4 h - x calls

The time is the same, so it's irrelevant.
1 volunteer - 48 calls
12 volunteers - x calls

x=12*48=576

So it's 576 hours.