Qisi has 20 yuan, and each pack of biscuits is 5 yuan. How much should you get back when you buy 3 packs of biscuits?
奇思有20元，每包饼干5元，买3包饼干应找回多少元．
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<result>5</result>