I was never really able to solve this problem.
Can you help me?
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?
If they were painting different houses at the same time,
Sally will have painted one and a half houses in 6 hours,
while John will have painted one.
Hence, they will have painted two and a half houses in 6 hours, while painting at the same time.
That's 5 houses in 12 hours.
So, it will take a fifth of that time to paint one house, working together.