21 ways
21 ways
5 ways
Opposite faces sum to 21.
21 ways.
It depends on how many shelves there are, which you did not specify. Please restate the question. Assuming that there are just 2 shelves with 21 books on each, then the 21 books on the top shelf can be arranged in factorial 21 different ways, with the same result for the lower shelf. To find "factorial 21", which is written as 21! the calculation required is 21 x 20 x 19 x 18....and so on all the way down to ...x 3 x 2 x 1! However, with access to a scientific calculator the result for 21!can be found by first pressing the buttons for 21 followed by pressing the button labelled n!, which gives the result for 21! as 5109094217109440000 for each shelf. The result for the two shelves is 21! x 21! which is such a tremendous number that no-one could arrange books in that way if they spent their whole life at it (including from birth).
21
21
Many people..........
60%-70%..of people..!
21
21 billion