Dave paints a fence in 4 hours while Sara paints the same fence in 2 hours. If they work together, h

[math_celebrity]

Dave paints a fence in 4 hours while Sara paints the same fence in 2 hours. If they work together, how long will it take them to paint the fence?

Set up unit rates:

• Dave paints 1/4 of the fence in 1 hour
• Sara will paint 1/2 of the fence in 1 hour

So together, they paint 1/2 + 1/4 = 2/4 + 14 = 3/4 of the fence in one hour.
1 hour = 60 minutes, so we set up a proportion of time to minutes where m is the time in minutes needed to complete 1 full fence:
3/4/60 = 1/m
3/240 = 1/m

Typing this proportion in our math engine, we get:
m = 80 minutes

80 minutes is also 1 hour and 20 minutes.