Question 457324
Since they all have to mix 20 drinks, I can call
20 drinks 1 job
Let {{{t}}} = time for  all 3 working together 
to mix 20 drinks
given:
Stephen's rate of working =  (1 job) / (5 min)
Sue's rate = (1 job) / (10 min)
Jack's rate = (1 job) / (15 min)
Add their rates
{{{ 1/5 + 1/10 + 1/15 = 1/t }}}
Multiply both sides by {{{30t}}}
{{{ 6t + 3t + 2t = 30 }}}
{{{ 11t = 30 }}}
{{{ t = 2.73 }}}
{{{ .73*60 = 44 }}}
It will take them 2 minutes and 44 seconds to mix 20 drinks