Question 1128711
Add their rates of working to get 
their rate working together
Let {{{ x }}} = the fraction of the
plot they hoe working together
in 1 hr
{{{ 1/5 + 1/4 = x/1 }}}
Multiply both sides by {{{ 20 }}}
{{{ 4 + 5 = 20x }}}
{{{ 20x = 9 }}}
{{{ x = 9/20 }}}
They hoe 9/20 of the plot in 1 hr woking together
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Let {{{ t }}} = time in hrs to hoe 1 plot
working together
{{{ 1/5 + 1/4 = 1/t }}}
Multiply both sides by {{{ 20t }}}
{{{ 4t + 5t = 20 }}}
{{{ 9t = 20 }}}
{{{ t = 20/9 }}} hrs
20/9 hrs working together