A herd of bison lived in the forest. They were about to move. The bison king called the bison together. When he called the bison (he also counted), he found that the ratio of the number of absent animals to the attendance index was 1:3. He was very angry. When he wanted to criticize, two more came. The ratio of absent to present is 1:4. How many bison are there?
一群野牛居住在森林里．它们要迁居，野牛王把野牛召集在一起，点名时(自己也点)，发现缺席只数与出席指数的比是1：3，他很生气，要批评时，又跑来2只，这时缺席只数与出席只数的比是1：4，问有野牛共多少只？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">2 / (1/20)</gadget>
<output>40</output>

<result>40</result>