Question 477420
Add their rates of working
Rate of 1st man = ( 1 job ) / ( 5 days )
Rate of 2nd man = ( 1 job ) / ( 8 days ) 
Let {{{x}}} = the number of days that  both men
working together take to do job
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{{{ 1/5 + 1/8 = 1/x }}}
Multiply both sides by {{{ 40x }}}
{{{ 8x + 5x = 40 }}}
{{{ 13x = 40 }}}
{{{x = 3.077 }}}
They can do the job in just over 3 days