Question 946331
Shawna's rate of cleaning:
( 1 attic ) / ( 13.3 hrs )
Cody's rate of cleaning:
( 1 attic ) / ( 12.3 hrs )
Cleaning together:
( 1 attic ) / ( t hrs )
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Add their rates to get rate
cleaning together
{{{ 1/13.3 + 1/12.3 = 1/t }}}
Multiply both sides by 
{{{ 13.3*12.3*t }}}
{{{ 12.3t + 13.3t = 12.3*13.3 }}}
{{{ 25.6t = 163.59 }}}
{{{ t = 6.3902 }}}
{{{ .3902*60 = 23.414 }}}
{{{ .414*60 = 24.843 }}}
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Working together, it takes them
6 hrs 23 min 25 sec
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check:
{{{ 1/13.3 + 1/12.3 = 1/6.3902 }}}
{{{ .07519 + .0813 = .15649 }}}
{{{ .15649 = .15649 }}}
OK