Question 967061
Add their rates of working
Let {{{ t }}} = time it takes Sam to do job
[ 1 job ] /  [ t hrs ] = rate for Sam
[ 1 job ] / [ t - 4 hrs ] = rate for Pete
[ 1 job ] / [ 16 hrs ] = their rate working together
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{{{ 1/t + 1/( t-4 ) = 1/16 }}}
Multiply both sides by {{{ t*( t-4 )*16 }}}
{{{ 16*( t-4 ) + 16t = t*( t-4 ) }}}
{{{ 16t - 64 + 16t = t^2 - 4t }}}
{{{ t^2 - 36t + 64 = 0 }}}
Use quadratic formula
{{{ t = ( -b +- sqrt( b^2 - 4*a*c )) / (2*a) }}} 
{{{ a = 1 }}}
{{{ b = -36 }}}
{{{ c = 64 }}}
You can finish. Find {{{ t }}}
and {{{ t + 4 }}}