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Seven boxes, D, E, F, R, S, T and U, are kept one over the other but not necessarily in the same order. U is kept immediately above T. E is kept immediately above D. Only F is kept above S. Only two boxes are kept above U. E is not kept at third position from the bottom. How many boxes are kept between R and F?
Four
Two
One
Three
Three
Fix F and S at top (positions 7 and 6), then U at 5th and T at 4th from top. D and E must fit in positions 1 and 2, placing R at 3rd. Simply count between F (pos 7) and R (pos 3): 7 - 3 - 1 = 3 boxes.
Fix F and S at top (positions 7 and 6), then U at 5th and T at 4th from top. D and E must fit in positions 1 and 2, placing R at 3rd. Simply count between F (pos 7) and R (pos 3): 7 - 3 - 1 = 3 boxes.
Step 1: Fix top positions from fixed clues
Total boxes = 7 (positions 1 to 7 from bottom to top). 'Only F is kept above S' implies F is at position 7 (topmost) and S is at position 6. 'Only two boxes are kept above U' means U is at position 5 (below F and S).
Step 2: Place U, T, and the adjacent pair E, D
'U is kept immediately above T', so T is at position 4. We now have positions 1, 2, and 3 remaining for E, D, and R. 'E is kept immediately above D' requires 2 consecutive available spots. Positions 1 and 2 are consecutive, so D is at position 1 and E is at position 2. This satisfies 'E is not kept at third position from the bottom' since E is at 2nd position.
Step 3: Determine position of R and count boxes between R and F
The remaining position 3 is occupied by Box R. The final order from Top (7) to Bottom (1) is: F (7), S (6), U (5), T (4), R (3), E (2), D (1). The boxes between R (position 3) and F (position 7) are T, U, and S, which gives a total count of 3 boxes.
A: Four is incorrect because there are only 3 boxes (S, U, T) between R and F. B: Two is incorrect because counting S, U, T gives 3 boxes. C: One is incorrect as it underestimates the count.
D is correct because the final arrangement from top to bottom is F-S-U-T-R-E-D, placing exactly 3 boxes (S, U, T) between R and F.
In vertical stacking puzzles, always look for absolute top/bottom conditions first (e.g., 'Only F is above S') to lock extreme positions immediately.