Question 578104
Add their rates of working to get
 rate of working together
Tom's rate: ( 1 yard ) / ( 30 min )
Jerry's rate: ( 1 yard ) / ( 40 min )
Let {{{ t }}} = time in minutes to mow yard working together
Their rate working together: ( 1 yard ) / ( t min )
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{{{ 1/30 + 1/40 = 1/t }}}
Multiply both sids by {{{ 120t }}}
{{{ 4t + 3t = 120 }}}
{{{ 7t = 120 }}}
{{{ t = 120/7 }}} min
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All these type of problems are about adding ( sometimes subtracting ) rates