29) B

Draw out a string after every step of the question.

Another way to do this is:
We know the red mark would be at the half of the string = 1/2
The green marks would be at the half points of the half marks = 1/4 and 3/4
The blue marks would be at the half of the half of the half points = 1/2 x 1/4 = 1/8. i.e. at every 1/8 point. Blue marks would be at: 1/8, 3/8, 5/8, 7/8
Purple marks would be at 1/16, 3/16, 5/16, 7/16, 9/16, 11/16, 13/16, 15/16

We know the green marks are present at 1/4 and 3/4. We need to find the number of points that are larger than 1/4 but smaller than 3/4.
1/4 = 2/8 = 4/16
3/4 = 6/8 = 12/16
The points are: 5/16, 3/8, 5/8, 7/16, 9/16, 11/16, 1/2

Number of points between green marks = 7. Option B

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