Question 559531
Let {{{ t }}} = minutes it takes Sue to do the route by herself
Tim's rate of working = ( 1 paper route ) / ( 120 minutes )
Sue's rate of working= ( 1 paper route ) / ( t minutes )
Rate working together = ( 1 paper route ) / ( 90 minutes )
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Add their rates of working to  get rate working together
{{{ 1/120 + 1/t = 1/90 }}}
Multiply both sides by {{{ 360t }}}
{{{ 3t + 360 = 4t }}}
{{{ t = 360 }}}
It takes Sue 360 minutes to do the route by herself