IT takes Holly 5 hours to type a paper. It takes Mike 6 hours to type the same paper. HOw long would it take them working together? Just average?
clearly not just the average, since that would be 5.5, or slower than Holly by herself.
You have to consider how much works gets done in 1 hour by each person. Then you can add that up to see how much gets done together.
1/5 + 1/6 = 11/30
so, the paper takes 30/11 hours for them together. Or, just under 3 hours.
Makes sense, since two Hollys could do it in 2.5 hours, and two Mikes would take 3 hours.