Question 281166
Before we actually solve it let's look at what you tried.
8/6 does not equal .75 but  4/3 or 1 1/3
6/8=.75
and you are right 12 painters should do it faster than 6 hours unless the new painters are all supervisors.

Now to solve it.
8 painters worked for 6 hours
That's 48 manhours were needed
The number of manhours is the same but we are going to divide 48 by 12 workers. not just 8
48/12=4 
It should take the 12 painters 4 hours if they have their own equipment and don't get in each others way.

6*8=x*12
48=x*12
4=x
what is 4? hours worked by 12 workers
what is 6? hours worked by 8 workers