Clues

1. Four books P, Q, R, and S are placed on a shelf.
2. P is at the left end.
3. Q is not next to P.
4. R is immediately before S.

Question

What must be the order from left to right?

Answer

P, R, S, Q.

Solution

1. P is fixed in the first position.
2. Q cannot be second, and R-S must be consecutive.
3. The only arrangement is P, R, S, Q.