The apple is 4 yuan per catty. Mom bought 7 catties of apples and paid the salesperson 50 yuan. How much should I get back?
苹果4元1斤，妈妈买了7斤苹果，付给售货员50元钱，应找回多少钱？
<gadget id="calculator">4 * 7</gadget>
<output>28</output>

<gadget id="calculator">50 - 28</gadget>
<output>22</output>

<result>22</result>