tea set shop stipulates that every purchase of a teapot will give a teacup as a gift. It is known that each teapot is 15 yuan and each teacup is 2.5 yuan. Mr. Li bought 4 teapots and some teacups for 75 yuan. How many teacups did Mr. Li buy back?
某茶具店规定：凡购买一个茶壶赠送一个茶杯．已知每个茶壶15元，每个茶杯2.5元．李老师购买了4个茶壶和一些茶杯共用了75元，李老师买回了多少个茶杯？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">75 - 60</gadget>
<output>15</output>

<gadget id="calculator">15 / 2.5</gadget>
<output>6</output>

<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<result>10</result>