Question 885105
Since it's only about washing 5 cars, call that 1 job
1st person's rate of washing:
( i job ) / ( 36 min )
2nd person's rate of washing:
( 1 job ) / ( 42 min )
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Let {{{ t }}} = the time to finish job
working together
Add their rates of working to get 
their rate working together
--------------------------
{{{ 1/36 + 1/42 = 1/t }}}
{{{ ( 1/6 )*( 1/6 + 1/7 ) = 1/t }}}
{{{ 1/6 + 1/7 = 6/t }}}
Multiply both sides by {{{ 42t }}}
{{{ 7t + 6t = 6*42 }}}
{{{ 13t = 252 }}}
{{{ t = 19.3846 }}}
It will take them 19 and 5/13 min woking together
check:
{{{ 1/36 + 1/42 = 1/t }}}
{{{ .027777 + .023809 = 1/19.3846 }}}
{{{ .051587 = .051587 }}}
OK