There are three colored glass balls, red, yellow and blue. The total weight of red balls and yellow balls is 20, the total weight of yellow balls and basketballs is 18, and the total weight of red balls and basketballs is 12. How many of the three kinds of balls are there in total?
有红、黄、蓝三种颜色的玻璃球，红球和黄球共重20个，黄球和篮球共重18个，红球和篮球共有12个，三种球一共多少个．
<gadget id="calculator">20 + 18 + 12</gadget>
<output>50</output>

<gadget id="calculator">50 / 2</gadget>
<output>25</output>

<result>25</result>