Question 1025945
First of all, you have to imagine 2 people working
on an engine without getting in eachother's way.
It's not like painting a fence.
But as long as you can assume that, add their
rates of working to get their rate working together.
BJ's rate:
[ 1 engine overhauled ] / [  20 hrs ]
Apprentice's rate:
[ 1 engine overhauled ] / [ 6 hrs ]
Let {{{ t }}} = their time in hrs working together
to overhaul engine
Rate working together:
[ 1 engine overhauled ] / [ t hrs ]
------------------------------
{{{ 1/20 + 1/6 = 1/t }}}
Multiply both sides by {{{ 60t }}}
{{{ 3t + 10t = 60 }}}
{{{ 13t = 60 }}}
{{{ t = 4 + 8/60 }}}
Working together, it takes them 4 hrs and 8 min
check:
{{{ 1/20 + 1/6 = 1/t }}}
{{{ 1/20 + 1/6 = 1/(60/13) }}}
{{{ 1/20 + 1/6 = 13/60 }}}
{{{ 3/60 + 10/60 = 13/60 }}}
{{{ 13/60 = 13/60 }}}
OK