The school buys 15 boxes of colored chalk, 50 sticks in each box, and 10 boxes are used. How many branches are left useless?
学校买15盒彩色粉笔，每盒50枝，用去10盒．还剩多少枝没有用？
<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<gadget id="calculator">5 * 50</gadget>
<output>250</output>

<result>250</result>