I am having a hard time with these word equations. Are teacher goes way to fast and I am trying my best to understand him and take notes but I still can't figure out how to set these up and I think if I can figure that out than I'll be able to follow through and figure these word problems out. But I am at a total loss. I can't even get past #1 , I have gone over and over with the examples in the book but I'm still lost.

Thanks for the help if you can.

Juan can paint a 3600 - square foot house in 12 days, and Carissa can do the same job in 8 days. How long will it take them to paint a 3600 square foot house if they worked together?