Fred and Sally are going to paint a house. Fred knows that if he had to paint the house by himself it would take him 2 times as long as it would take Sally to paint the house by herself. If they work together, they can have the house painted in 24 hours.
Which of the following equations could help us determine how long it would take Sally to paint the house on her own?

24/x + 24/2x = 1
x + 2x = 24
1/x + 1/2x = 1
x/24 + 2x/24 = 1