Answer
20/9 hours
Work Step by Step
experienced surveyor: finishes in 4 hours (or 1/4 of the job in an hour)
apprenticed surveyor: finishes in 5 hours (or 1/5 of the job in an hour)
$1/4 + 1/5 = 1/x$
$20(1/4 + 1/5 = 1/x)$
$5 + 4 = 20/x$
$9=20/x$
$9*x*1/9 = 20/x *x*1/9$
$x = 20/9$
