In this problem 10 represents 9% of a number. So, to find 90% you must multiply 10 by 10. You do this because 9% times 10 equals 90%; therefore, 100 must be 90%.
This can also be proven through cross multiplication. To set up a proportion put the percent over the number like, . Then cross multiply to get the equation, . Finally, divide both sides to get x=100.