Li Hua bought a straw hat for 1.8 yuan, bought a teapot for 2.5 yuan, and bought a hot water bottle. The three items cost exactly 9 yuan. How much does it cost to buy a hot water bottle?
李华买一顶草帽用1.8元，买一把茶壶用2.5元，又买了一个热水瓶，三样东西正好用了9元．买热水瓶用多少元？
<gadget id="calculator">9 - 1.8 - 2.5</gadget>
<output>4.7</output>

<result>4.7</result>