Xiaoping and Xiaoli went to Xinhua Bookstore to buy books. They chose the same book, but they did not bring enough money. Xiaoping was short of 15 yuan, and Xiaoli was 2 yuan short. They had to buy a book together, which was 1 yuan more. How much is each book.
小平和小丽到新华书店去买书，她们选中了同一本书，可是她们带的钱不够，小平差15元，小丽差2元，只好先合买一本，还多1元．每本书多少元．
<gadget id="calculator">15 + 2 + 1</gadget>
<output>18</output>

<gadget id="calculator">1 - 1</gadget>
<output>0</output>

<gadget id="calculator">1 + 0</gadget>
<output>1</output>

<gadget id="calculator">18 / 1</gadget>
<output>18</output>

<result>18</result>