A positive number is 5 times another number, if 21 is added to both numbers, then one of the new numbers becomes twice the other new number, what are the numbers?

x - the first number,

y - the second number

We have:

So answer is 7 and 35.

You can stack the eight books on top of each other is one way. And then you can changed the order of how you stack the book 64 different ways. You can also do this vertically standing up. With the spin of the book facing in or out. You can arrange the. By height, color, alphabetically, thickness, year published... basically any individual characteristic they can have can be a way you stack a book.