70% of what number is 21 and how do you do it

70 % of unknown number = 21
We multiply the percentage by a variable and make it = to the given value.
70% of n = 21
Change percent to decimal
70% =  = 0.7
0.7n = 21
divide either sides by 0.7 to isolate n

= 
0.7 and 0.7 cancels out
n = 30

0.7 × 30 = 21
70 % of 30 is 21
An example has been attached below

% of a number = multiply % by number
value is the % of a number = divide % by value

Depends on the class. I'm almost done with my math for the semester.