Amason and his assistant work together for 6 hours on a brick fireplace before the mason has to leave the job. the assistant finishes the job alone in 12 hours. if the mason can construct a fireplace in 15 hours working alone, how long does it take his assistant working alone to construct a fireplace?

The trend in the equation seems that the time that the assistant works is double the time of how long the mason works on his own. In the equation it looks like this:

6 hours (mason) X 2 (assistant) = 12 hours work 
15 hours (mason) x 2 (assistant) = 30 hours work

This is probably because the assistant is less experienced compared to the mason.