Answer
2 hours
Work Step by Step
experienced: completes job in 3 hours (or 1/3 of job in an hour)
apprentice: completes job in 6 hours (or 1/6 of job in an hour)
$1/3 + 1/6 = 1/x$
$6(1/3 + 1/6 = 1/x)$
$2 + 1 = 6/x$
$3 = 6/x$
$3*x/3 = 6/x *x/3$
$x = 2$
$1/3 + 1/6 = 1/x$
$1/3 + 1/6 = 1/2$
$2/6 + 1/6 = 1/2$
$3/6 = 1/2$ (true statement)