Question 1108568
Jessica's rate of washing:
[ 1 dog washed ] / [ 45 min ]
Brother's rate of washing:
[ 1 dog washed ] / [ 30 min ]
Their rate working together:
[ 1 dog washed ] / [ t min ]
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Add their rates of washing to get rate 
washing together
{{{ 1/45 + 1/30 = 1/t }}}
Multiply both sides by {{{ 90t }}}
{{{ 2t + 3t = 90 }}}
{{{ 5t = 90 }}}
{{{ t = 18 }}}
It will take them 18 min working together