Hi, I wasn't sure how to approach this problem: You have 5 differently colored stones-red, orange, blue, green, purple. If the green stone cannot be placed at the front or the back of the sequence, how many possible arrangements can you make? I know that without the above restriction, the amount would be 5!=120. But I don't get how to use the restriction. BTW the back of the book says that the answer is 72. Help please! thanks.