The problem here is actually not exactly a mathematical problem, it is rather a language problem. If you understand the meaning of the problem, actually this is not so much math in there.
Since the question is described in a language (originally in Japanese, here in English. I found it is interesting this worked for both languages), we need to understand what the problem means. Here we need to know what is the 'goal success ratio' and what does 'get together' means. I wanted to him to think about these words really means. A goal success ratio is actually simple. It is the ratio of how many shoots were tried and how many goals were succeeded. But I didn't realized for a long time, I found people actually don't think about what is 'get together' means. When I found this, I made this problem. Here I made up an expression that adds two numbers together, but the result doesn't change. This is a strange calculation, isn't it? I use the double meanings of the 'get together' to make this happen.
I coincidentally find that the words 'get together' is actually not a simple expression. Language is a deep subject. The phrase is used so often, we are just too familiar with that. But sometimes I can find something so familiar, thus I thought I knew it well, but I actually didn't know about it. Maybe similar things happens with a person. I think I know someone well, but one day I realized I didn't know that person well... This is a blind spot. If you learn literature, you may deeply think of each word you are using. However it is hard to do in the everyday life. But if you don't realized what is wrong here, that means you may think that adding non-zero numbers don't change the amount. Someone could use this method as a fee calculation or a tax calculation. It seems you didn't pay much at the amount, but you actually pay a lot. I don't want to people are deceived by this kind of wrong logic. Since I am living in a democratic country, if many people are deceived, I will also deceived. Actually I am forced to be deceived. That's democracy.
This boy understood the meaning of the question. And he found my lie. Then, he asked me, ``Does a teacher lie?'' This was an excellent question. I answered, ``Of course! A teacher would lie. But, most of the cases, a teacher actually doesn't lie, but a teacher may make a mistake. If you could find that by thinking yourself, I am really glad.'' (I also hope he has a good communication skill since I had a lot of trouble of lacking the skill.)
Any authority, like a teacher, might lie and might make a mistake. Many of my generation knows words: ``We, adults, don't lie on purpose, but we make mistakes.'' when a famous cartoonist in Japan made a fatal mistake in his story. The boy realized it. I wish he continues to try to understand the world himself, not just following the authority. I just teach him one method to think himself through mathematics. We could also think through learning literature, or foreign languages, and so on. But I think it is a good start. I am so glad he found this by himself.
Nowadays, our life is deeply based on science. Thus I believe there is a basic science knowledge we, all the citizen, should know about it. In many countries, everyone needs to learn some science in the elementally school. I think this is not for making everyone scientists. We need to know the infrastructure of our society. For instance, we cannot completely ignore where the electricity comes from. It may affect the environment, and I recall the Fukushima incident. We should know some concept of computer security, otherwise we cannot safely buy anything through the internet. I believe we all need some basic knowledge about math and science in our society. I will be happy if my article is one of the clues for someone who later become curious to science or mathematics.
In this article, I didn't exactly explain what is the problem in the question. But 13 year-old boy can find it. I also put some hints in the article, so I hope you think through the problem. Please don't search it immediately. Let's think first before searching the answer on the internet.