Joey and rita can individually paint a room in 8 hours. it takes billy 12 hours to paint this same room. if all three work together, how long would it take them to paint this room?

We know that
Joey and Rita paint a room in 8 hours
so
100%> 8 hours
 X%> 1 hour
x=100/8> 12.5%  
that means 
Joey and Rita paint 12.5% of a room in 1 hour

Billy paint a room in 12 hours
so
100%> 12 hours
X %> 1 hour
x=100/12> x=8.33 %
that means 
Billy paint 8.33% of a room in 1 hour

if all three work together
then
All three paint (12.5%+8.33%) in 1 hour

20.83%> 1 hour
100%> x
x=100/20.83> x=4.8 hours

the answer is
4.8 hours