Question 1018913
Let {{{ t }}} = their time in minutes working together
Add their rates of working to get their rate working together
Fred's rate:
[ 1 lawn ] / [ 120 min ]
Rocky's rate:
[ 1 lawn ] / [ 80 min ]
Rate working together:
[ 1 lawn ] / [ t min ]
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{{{ 1/120 + 1/80 = 1/t }}}
Multiply both sides by {{{ 240t }}}
{{{ 2t + 3t = 240 }}}
{{{ 5t = 240 }}}
{{{ t = 48 }}}
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It takes them 48 min to mow 
lawn working together
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check:
{{{ 1/120 + 1/80 = 1/t }}}
{{{ 1/120 + 1/80 = 1/48 }}}
{{{ 1/30 + 1/20 = 1/12 }}}
{{{ 1/15 + 1/10 = 1/6 }}}
{{{ .0667 + .1 = .1667 }}}
{{{ .1667 = .1667 }}}
OK