Question 1039721
1st student's rate of working:
( 1 job ) / ( 10 min )
2nd student's rate of working:
( 1 job ) / ( 20 min )
Let {{{ t }}} = time in minutes for them 
to do the job working together
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Add their rates of working to get their rate
working together
{{{ 1/10 + 1/20 = 1/t }}}
Multiply both sides by {{{ 20t }}}
{{{ 2t + t = 20 }}}
{{{ 3t = 20 }}}
{{{ t = 6.667 }}} min
Convert {{{ .667 }}} min to seconds
{{{ .667*60 = 40 }}} sec
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Working together, they finish the job
in 6 min 40 sec