Obscene asked:
Please give your answer AND explanation please.
Please give your answer AND explanation please.
In the bookshop, assistants are preparing for a famous author to launch his new title, Economy: Rising and Falling.
Copies are piled up flat on a table, in four equal stacks. From one side, only 9 spines in each stack are visible because they are piled up alternately: in other words, any book lying on top of another will have its spine in exactly the opposite direction to that of the book below it.
What is the least possible number of copies on the table?