Mom has 100 yuan and bought 2 footballs, each of which costs 40 yuan. How much money does mom have left?
妈妈有100元钱，买了2个足球，一个足球40元．妈妈还剩下多少元？
<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<result>20</result>