Question

21 is what percent of 42

Answer 1

21 is 50% of 42. 
42 x 0.5 = 21

Answer 2

1. We assume, that the number 42 is 100% - because it's the output value of the task.
2. We assume, that x is the value we are looking for.
3. If 100% equals 42, so we can write it down as 100%=42.
4. We know, that x% equals 21 of the output value, so we can write it down as x%=21.
5. Now we have two simple equations:
1) 100%=42
2) x%=21
where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
100%/x%=42/21
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 21 is what percent of 42

100%/x%=42/21
(100/x)*x=(42/21)*x       - we multiply both sides of the equation by x
100=2*x       - we divide both sides of the equation by (2) to get x
100/2=x
50=x
x=50

now we have:
21 is 50% of 42