Using the work word problems, 1/3 + 1/6 = 2/6 + 1/6
3/6 = 2
setting 1/x, where 1 is a completed job, then x/1 = 2/1, therefore, the combined effort finishes the job in 2 hours. This is from the best of my knowledge. Purple Math is a great resource if extra help is needed.
