Question 1091243
Mark's rate is {{{ 2/m }}}
Bob's rate is {{{ 2/b  }}}
Add their individual rates to get 
their rate working together
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Let {{{ t }}} = their time in hrs to do 1 job working together
{{{ 2/m + 2/b = 1/t }}}
Multiply both sides by {{{ m*b*t }}}
{{{ 2*b*t + 2*m*t = m*b }}}
{{{ ( 2b + 2m )*t = m*b }}}
{{{ t = ( m*b )/( 2b + 2m ) }}}