Roland and Sam wash dogs to make extra money. Roland can wash all of the dogs in 4 hours. Sam can wash all of the dogs in 3 hours. How long will it take them to wash the dogs if they work together?
If Roland can wash all of the dogs in 4 hours, then in one hour he can wash 1/4 of the dogs.
Similarly, if Sam can wash all of the dogs in 3 hours, then in one hour he can wash 1/3 of the dogs.
When they work together, they can wash 1/4 + 1/3 = 3/12 + 4/12 = 7/12 of the dogs in one hour.
Therefore, it will take them 12/7 hours to wash all of the dogs if they work together. Answer: \boxed{1\frac{5}{7}}\text{ hours}.