Five books P, Q, R, S and T are placed one above the other in a single stack. R is immediately above S. Q is immediately above R. T is at the bottom of the stack. P is not at the top. Which book is at the top?

Number the places 1 at the bottom to 5 at the top. T takes place 1. Since R is just above S and Q is just above R, the three books S, R, Q sit in three places one after the other, and P takes the place left over. If S, R, Q filled places 2, 3, 4 then P would be at place 5, the top, which is not allowed. So S, R, Q fill places 3, 4, 5 and P takes place 2. The book at the top, place 5, is Q.