Each pen is 4 yuan. Xiaohong bought 5 pens and paid 50 yuan. How much will she get back?
每支钢笔4元，小红买了5支钢笔，付了50元，要找回多少元？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">50 - 20</gadget>
<output>30</output>

<result>30</result>