Today's apple is 4.5 yuan/kg, and my father bought 3.5 kilograms. How much should I get back after my father paid 50 yuan.
今天苹果4.5元/千克，爸爸买了3.5千克，爸爸付了50元应找回多少元．
<gadget id="calculator">4.5 * 3.5</gadget>
<output>15.75</output>

<gadget id="calculator">50 - 15.75</gadget>
<output>34.25</output>

<result>34.25</result>