Question 1032312
There is a requirement of 15*12 = 180 person-days to finish repairing the highway.  Since it has been 4 days that the crew has been working, 4*15 = 60 person-days have already been accomplished.  The remaining work is done by 15+x persons in 6 days.  Hence, 

60 + 6(15+x) = 180

==> 6(15+x) = 120
==> 15+x = 20 
==> x = 5.
Therefore five new workers joined the crew.