A football in the store is 29 yuan. Xiao Ming buys 3 footballs and pays the salesperson 100 yuan. How much should he get back?
商店里一个足球29元，小明买3个足球，付给售货员100元，应找回多少元？
<gadget id="calculator">29 * 3</gadget>
<output>87</output>

<gadget id="calculator">100 - 87</gadget>
<output>13</output>

<result>13</result>