There are 150 science books in a class, and 50 story books are twice as many as science books. How many story books are there?
某班存放科技书150本，故事书比科技书的2倍少50本，故事书有多少本？
<gadget id="calculator">150 * 2</gadget>
<output>300</output>

<gadget id="calculator">300 - 50</gadget>
<output>250</output>

<result>250</result>