"If Steven can mix 20 drinks in 5 minutes, Sue can mix 20 drinks in 10 minutes, and Jack can mix 20 drinks in 15 minutes, how much time will it take all 3 of them working together to mix the 20 drinks?

is there a rule for this??!"

Well Steven is mixing 20/5= 4 drinks per minute.

Sue is mixing 20/10=2 drinks per minute.

Jack is mixing 20/15=4/3 drinks per minute.

So in 1 minute (60 seconds), the 3 of them can mix 4+2+(4/3)=(22/3) drinks

So:

60 seconds - 22/3 drinks

90/11 seconds - 1 drink

(90/11)*20 seconds = 20 drinks

So the answer is 163.6 seconds.

There is probably a quicker way though