Xiaohua went to the store to buy a pencil case and 3 ballpoint pens, which cost 7.8 yuan. It is known that a pencil case costs 4.2 yuan. How much is each ballpoint pen? (solution by equation)
小华到商店买一个文具盒和3支圆珠笔，共用了7.8元，已知一个文具盒是4.2元，每支圆珠笔是多少元？(用方程解)
<gadget id="calculator">7.8 - 4.2</gadget>
<output>3.6</output>

<gadget id="calculator">3.6 / 3</gadget>
<output>1.2</output>

<result>1.2</result>