Seven people C, D, E, F, G, H and I are sitting in a straight row, facing…

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Seven people C, D, E, F, G, H and I are sitting in a straight row, facing north. D sits fifth to the left of I. C sits fifth to the left of H. Only three people sit to the right of F. H sits second to the right of F. E sits third to the left of H. Who sits third to the right of D?

Answer: C. GConcept: In a linear seating puzzle a fixed number of seats stand side by side in one line, and every clue is either an absolute anchor or a relative offset.…

  1. A.

    F

  2. B.

    H

  3. C.

    G

  4. D.

    E

Attempted by 31 students.

Show answer & explanation

Correct answer: C

Concept: In a linear seating puzzle a fixed number of seats stand side by side in one line, and every clue is either an absolute anchor or a relative offset. When the people face north, a person's right hand points toward the far end of the row as it is written from left to right, so "X sits n places to the right of Y" means the seat of X is n seats beyond the seat of Y, and "n places to the left" means n seats before it. The count always begins at the seat immediately next to Y, never at Y's own seat. A clue that states how many people sit on one side of a person is an absolute anchor, because it fixes that person's seat outright. The method is therefore to place the anchor first and then propagate every offset from it.

Application: Number the seven seats 1 to 7 from the left end of the row to the right end, so a higher seat number lies further to the right.

  1. Only three people sit to the right of F, so the three seats 5, 6 and 7 lie beyond F, which puts F in seat 4.

  2. H sits second to the right of F, so the seat of H is 4 + 2 = 6.

  3. C sits fifth to the left of H, so the seat of C is 6 - 5 = 1.

  4. E sits third to the left of H, so the seat of E is 6 - 3 = 3.

  5. D sits fifth to the left of I, so the seat of D is the seat of I minus 5. Seats 2, 5 and 7 are still free, and 7 - 5 = 2 is the pairing that fits inside them, so I takes seat 7 and D takes seat 2.

  6. One person, G, and one seat, number 5, are left over, so G takes seat 5.

Seat

Person

1

C

2

D

3

E

4

F

5

G

6

H

7

I

Result: D occupies seat 2, so the three seats to the right of D are 3, 4 and 5. The third of them is seat 5, and the person seated there is G. G therefore sits third to the right of D.

Cross-check: read the finished row C, D, E, F, G, H, I from left to right and test every clue against it.

  • D in seat 2 and I in seat 7 are five seats apart, so D is fifth to the left of I.

  • C in seat 1 and H in seat 6 are five seats apart, so C is fifth to the left of H.

  • F in seat 4 leaves three people, G, H and I, seated to the right of F.

  • H in seat 6 is two seats beyond F in seat 4, so H is second to the right of F.

  • E in seat 3 is three seats before H in seat 6, so E is third to the left of H.

A frequent slip is to count D's own seat as the first of the three steps, which stops one seat early; the count must begin at the seat next to D.

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