how do you do this algebra problem on work?

person A puts up wallpaper in the bedrooms of a house in 3 hrs. person B can do the same job in 4 hrs, and person C can do the same job in 12 hrs. if all three of them work together, how many minutes will it take them to put up the wallapaper?

notethese are easy to set upIn one hourA wallpapers 1/3 of the houseB wallpapers 1/4 of the house, andC wallpapers 1/12 of the houseif x = the number of hours to wallpaper one house, then1/3 x + 1/4 x + 1/12 x = 1multiply each term by the LCD = 124x + 3x + x = 12combine like terms8x = 12x = 12/8 = 3/2 = 1.5 hours = 90 minutes

A took: 180 minutesB took: 240 minutesC took: 720 minutesso the average time they take is : (180 + 240 + 720) / 3 = 380 minutes

=(3+4+12)/3=19/3