Teacher Wang bought 9 fountain pens and 9 ballpoint pens, and the pens cost 180 yuan to share. Ballpoint pen share to 45 yuan. How much is each fountain pen more expensive than a ballpoint pen?
王老师买了9支钢笔和9支圆珠笔，钢笔共用去180元．圆珠笔共用去45元．每支钢笔比圆珠笔贵多少元？
<gadget id="calculator">180 / 9</gadget>
<output>20</output>

<gadget id="calculator">45 / 9</gadget>
<output>5</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<result>15</result>