Nowadays, computer games are becoming more and more popular in many cities and towns.
   A lot of small shops along busy streets have been made into small game houses in order to get more money. These places are always crowded with people, especially young boys.
   In the computer game houses, people spend a lot of money competing（较量） with the machines. It's hard for one to win the computer, but one can make progress after trying again and again. People have a kind of gambling psychology（赌博心理）when they play computer games. The more they lose, the more they want to win. When class is over, the school boys run to the nearby computer game houses. Little by little（渐渐地）, they forget all about their lessons and fall far behind others. Some of them can get enough money from their parents. But some of them are not lucky enough to get the money. They begin to take away other students' money and become thieves.

$\frac{15}{3}$ =____          $\frac{30}{10}$ =____

 20℃时，向3个盛有50克水的烧杯中，分别加入一定质量的氯化钠，并充分溶解。根据下表实验数据回答。