20 is what percent of 50?

A. 30%

B. 10%

C. 250%

D. 40%

We assume, that the number 50 is 100% - because it's the output value of the task.

We assume, that x is the value we are looking for.

If 100% equals 50, so we can write it down as 100%=50.

We know, that x% equals 20 of the output value, so we can write it down as x%=20.

Now we have two simple equations:

1) 100%=50

2) x%=20

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%=50/20

Now we just have to solve the simple equation, and we will get the solution we are looking for.

100%/x%=50/20

(100/x) * x = (50/20) * x

100=2.5*x

100/2.5=x

40=x

x=40

The answer would be D.