Question 539754
Add their rates of mixing drinks to get 
their rate working together
Call mixing 20 drinks ( 1 job )
Steven's rate is ( 1 job ) / ( 5 min )
Sue's rate is ( 1 job ) / ( 10 min )
Jack's rate is ( 1 job ) / (15 min )
Let {{{ t }}} = time for all 3 working
 together to do 1 job
{{{ 1/5 + 1/10 + 1/15 = 1/t }}}
Multiply both sides by {{{ 30t }}}
{{{ 6t + 3t + 2t = 30 }}}
{{{ 11t = 30 }}}
{{{ t = 2.73 }}}
{{{ .73*60 = 43.6 }}}
It will take them 2 min and 44 sec