Xiao Ming went to the supermarket and bought a pen and a notebook for 20 yuan. It is known that the ratio of the unit price of the pen to the unit price of the notebook is 3:1, so what is the unit price of a pen?
小明到超市花20元买了一支钢笔和一个笔记本，已知钢笔的单价与笔记本的单价之比是3：1，那么一支钢笔的单价是多少元．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<result>15</result>