Make Job/Time ratios and add up the rates.

Let mixing 20 drinks be a job.

Then Steven does 1/5 jobs/min,

Sue does 1/10 jobs/min, and

Jack does 1/15 jobs/min

To find how long it will take them to make 20 drinks together, add up their rates.

we have 1/5 + 1/10 + 1/15 = 11/30 = 1/(30/11)

since the time to complete 1 job is in the denominator when 1 is in the numerator, they take 30/11 mins to complete the job together, that is roughly 2.7272 minutes