Rewrite the question, as follows:
45 is what percent of 225?
Form the ratio 45/225, and then multiply the result by 100%:
0.20(100%) = 20% (answer)
5, 10, 15, 20, 5n, 470
You just need to multiply the value by 5.
First you have all ten books, but then every time after, you loose one because you can't put the same book in the group twice.
10 * 9 * 8 * 7 = 5040 groups
