Answer:
The answer is D(20)
Step-by-step explanation:
I don't know the exact algebraic explanation, however, this is how I did it.

If you divide the second equation into seperate twos, you will get 2*2*2*2*2*2 = 
Therefore,
meaning that x is equal to 6. Now The next bit is trial and error, 20-6=14. 
Last thing to do is to add x+y=6+14. That is why the answer is 20.
If you wait a bit longer, someone will probably have a more algebraic explanation, however, you stated you only needed the answer so I hope this helped.
A is the answer because it doesn’t have a repeating x value
The formula of a volume of a cone:

We have r = 5m and H = 6m. Substitute:

1. False
2. There isn’t a variable, I cannot answer.