Suppose you and a friend can each dig a 20-foot ditch by yourselves in 3 hours. How long would it take you to dig the ditch working together, assuming you both have enough room to work without getting in each other's ways?

A little reasoning tells us that the project should get finished in half the time since you're doubling your efforts by combining forces, so it should take $1\frac12$ hours to dig the ditch.