Yeah, so I need help studying for my COMPASS test...

I imagine throughout random points when I get stuck, I’ll be posting another question in this thread. For right now… I wanna know what the hell is going on with this one.. There’s an answer key so I KNOW what the answer is. What I DON’T know is what formula I have to use to GET that answer. Obviously the actual questions will be different from these practice ones.

If Sally can paint a house in 4 hours, and John can paint the same house in 6 hour, how long will it take for both of them to paint the house together?