Diving1

　Today, I dived into the sea three times (at Maeda point one time and Yamada point twice). This diving is the course of "Adovance Open Water Diver" license. I have a "Open Water Diver" license already but I can not dive more than 18 meters depth by this license. And if I get a "Advanced Open Water Diver" license, I will be able to dive more than 18 meters (the max is 30 meters).
　Today's first diving was named "Deep diving". I dived 25 meters depth and this was my first experience. It is said that diver in deep get drank by Nitrogen. So, to make sure of this phenomenon, we tried to calculate some mathematics formula (like 42÷△＋12＝16 △＝？) from instructor. But almost all of us could solved them quickly. Then, I created the last question to the last challenger Kengo. My question was that "13×15÷7＝◯…△ △＝？". Kengo thought about it in many seconds and answered "1". I and Rikita said "Wrong". And we continued "3 is correct".After diving, we tried it again one the boat and we realized that the right answer is 6. We learned the phenomenon was true.