Ok here is how you do this:
you need to replace y with 20 and then solve to find x, i.e. solve this equation
20=2/5x
1. Simplify 2/5x to 2x/5
20=2x/5
2. Multiply both sides by 5
20 * 5=2x
3. Simplify 20 * 5 to 100
100=2x
4. Divide both sides by 2
100/2=x
5. Simplify 100/2 to 50
50=x
6. Switch sides
x=50
so your answer is B. 50
I hope this helped!