21 1/3 is called a mixed number. A mixed number is a whole number accompanied by a fraction. 21 is the whole number and 1/3 is the fraction. To turn that into a real fraction do the following. Multiply the whole number with the denominator of the fraction and add it to the nominator. So 21*3 equals 63. Add 1 to that and you'll get 64. The result is 64/3.
Now we've got 64/3 %. As I've explained in the answer to your other question ( http://bit.ly/9RxTuY
) this written as (64/3)/100. What we have here is called a complex fraction. A complex fraction is a fraction in which the nominator and / or the denominator itself are fractions.
To resolve this little problem we shall write the complex fraction a bit more complex. ;) As you'll remember 5 (just an example) is always 5/1. So 100 is 100/1. Our fraction is now (64/3)/(100/1).
To divide fractions you have the "multiply the dividend by the reciprocal of the divisor". Sounds difficult but it isn't. You can read more about this (and the proof) here: http://bit.ly/b0PFys
. Let's use variables for now. It's done like this: (a/b)/(c/d) = (a/b)*(d/c). Notice how c and d got switched. For our fraction this means (64/3)/(100/1) = (64/3)*(1/100).
Now, multiplying fractions is even easier. Just multiply the two nominators and multiply the two denominators. (64/3)*(1/100) = (64*1)/(3*100) = 64/300.
All we need to do now is to simplify the (new) fraction. But you already know how to do that. The result is 16/75.
I really suggest that you read the Wikipedia page on this subject (see link above). It's not difficult to understand and it even has a few nice cake examples. ;)