Cong Cong buys 2 books, "Encyclopedia for Children and Children" is 68.5 yuan, and "Little Pony's Adventure" is 17.8 yuan. Cong Cong paid two 50 yuan, how much should I get back?
聪聪买2本书，《少年儿童百科全书》是68.5元，《小马哥探险记》是17.8元．聪聪付了2张50元，应该找回多少元？
<gadget id="calculator">50 * 2</gadget>
<output>100</output>

<gadget id="calculator">100 - 68.5 - 17.8</gadget>
<output>13.7</output>

<result>13.7</result>