It takes Bill 4 hours to paint a wedding hall, and it takes Jessica 2 hours to paint the same hall. Working together, how many of these halls could they paint in 12 hours if they can continue painting at the same rate?
Bill can paint 1/4 of the hall in one hour because 1 / 4 = <<1/4=0.25>>0.25
Jessica can paint 1/2 of the hall in one hour because 1 / 2 = <<1/2=0.5>>0.5
Together, they can paint 0.75 of a hall in one hour because .25 + .5= <<.25+.5=0.75>>0.75
Together, they can paint 9 halls in 12 hours because 12 x 0.75 = <<12*0.75=9>>9. Answer: \boxed{9}.