We can find the answer to this question by first converting the statement into an equation.

Let's suppose that 21 is z percent of 81. So, we can say that:

z x 84 = 21

100

Now, we will simply solve this equation for z to obtain our answer:

z x 84 = 21

100

84z/100 = 21

84z = 21x100

84z = 2100

z = 2100/84

z = 25

Our answer:

21 is 25% of 84.

Let's suppose that 21 is z percent of 81. So, we can say that:

z x 84 = 21

100

Now, we will simply solve this equation for z to obtain our answer:

z x 84 = 21

100

84z/100 = 21

84z = 21x100

84z = 2100

z = 2100/84

z = 25

Our answer:

21 is 25% of 84.