Question 539747
Add their rates of working
Let {{{t}}} = their time working together
Jim's rate: ( 1 job ) / ( 30 min )
Sue's rate: (1 job ) / ( 45 min )
Tony's rate: ( 1 job ) / (90 min )
{{{ 1/30 + 1/45 + 1/90 = 1/t }}}
Multiply both sides by {{{ 90t }}}
{{{ 3t + 2t + t = 90 }}}
{{{ 6t = 90 }}}
{{{ t = 15 }}}
Working together, it will take them 15 min