Answer:
To find out what number 21 is 28% of, we can use the proportion:
21 / x = 28 / 100
where x is the unknown number we are trying to find.
We can solve for x by cross-multiplying and simplifying:
21 * 100 = 28 * x
2100 = 28x
x = 2100 / 28
x = 75
Therefore, 21 is 28% of 75.