Mr. Zhang bought two books. The science book cost 12.88 yuan. The encyclopedia cost 8.71 yuan more than the science book. Mr. Zhang gave the salesperson 50 yuan. How much should I get back?
张老师买了两本书，买科技书花了12.88元，百科全书比科技书多花8.71元，张老师给售货员50元，应找回多少钱？
<gadget id="calculator">12.88 + 8.71</gadget>
<output>21.59</output>

<gadget id="calculator">50 - 12.88 - 21.59</gadget>
<output>15.53</output>

<result>15.53</result>