Wang Li bought two books and spent a total of 28 yuan. The ratio of the unit prices of these two books is 3:4. How much does one cost more than the other?
王丽买两本书，共花28元，这两本书单价的比是3：4．其中一本比另一本贵多少钱？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(4/7) - (3/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">28 * (1/7)</gadget>
<output>4</output>

<result>4</result>