Question 514762
The big man can do 1/3 of a job in 4 hours
To do 3 times this amount of work, takes
him 3 times as long
So to do {{{ 3*(1/3) = 1 }}} job takes {{{ 4*3 = 12 }}} hrs
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The little man can do 1/2 a job in 2 hrs
and also {{{ 2*(1/2) = 1 }}} job in 4 hrs
So there is {{{ 30 - 1/2 = 29.5 }}} jobs left to do
Now the big man joins him
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Add their rates of working to get their rate
working together
{{{  1/12 + 1/4 = 29.5/x }}}
Multiply both sides by {{{ 12x }}}
{{{ x + 3x = 354 }}}
{{{ 4x = 354 }}}
{{{ x = 88.5 }}}
It takes them 88 and 1/2 hrs to finish the jobs 
working together