Question 1029143
Add their rates of mowing to 
get rate mowing together
Joe's rate:
[ 1 job / 8 hrs ]
Bob's rate:
[ 1 job / 3 hrs ]
Let {{{ t }}} = Tom's rate for the job
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{{{ 1/8 + 1/4 + 1/t = 1/(( 4/3)) }}}
{{{ 1/8 + 1/4 + 1/t = 3/4 }}}
( {{{ 4/3 }}} hrs = 1 hr 20 min )
Multiply both sides by {{{ 8t }}}
{{{ t + 2t + 8 = 6t }}}
{{{ 3t = 8 }}}
{{{ t = 2 + 2/3 }}}
Tom takes 2 hrs 40 min working alone
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check:
{{{ 1/8 + 1/4 + 1/t = 3/4 }}}
{{{ 1/8 + 1/4 + 1/((8/3)) = 1/((4/3)) }}}
{{{ .125 + .25 + .375 = .75 }}}
{{{ .75 = .75 }}}
OK