Question 1099742
Add their rates of washing to get their rate washing together
Fran's rate:
[ 1 job ] / [ 3 hrs ]
Jill's rate:
[ 1 job ] / [ 4.5 hrs ]
Working together:
[ 1 job ] / [ t hrs ]
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{{{ 1/3 + 1/4.5 = 1/t }}}
{{{ 1/3 + 2/9 = 1/t }}}
Multiply both sides by {{{ 9t }}}
{{{ 3t + 2t = 9 }}}
{{{ 5t = 9 }}}
{{{ t = 1.8 }}}
{{{ .8*60 = 48 }}}
Working together, they can wash the windows 
in 1 hr 48 min
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check answer:
{{{ 1/3 + 2/9 = 1/1.8 }}}
{{{ 1/3 + 2/9 = 10/18 }}}
{{{ 1/3 + 2/9 = 5/9 }}}
Multiply both sides by {{{ 9 }}}
{{{ 3 + 2 = 5 }}}
{{{ 5 = 5 }}}
OK