If we graph Mary's indifference curve with good x1 on the horizontal axis and good x2 on the vertical axis, then whenever Mary has more x2 than x1, the indifference curve's slope is -2. Whenever Mary has more x1 than x2, the indifference curve is -1/2.

Therefore, Mary would be indifferent between a bundle (23,29) and (31,X):

1. X =19
2. X = 22
3. X = 25
4. X = 23.5
5. X =27