There are 30 turtles and cranes in a wetland. There are 100 turtle legs and crane legs. How many turtles are there?
一块湿地上有龟、鹤共30只，龟的腿和鹤的腿共100条，龟有多少只．
<gadget id="calculator">4 * 30</gadget>
<output>120</output>

<gadget id="calculator">120 - 100</gadget>
<output>20</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<result>20</result>