Wang Liang and Li Feng went shopping in a stationery store together. Wang Liang bought 5 exercise books and 4 pencils and paid a total of 17.3 yuan. Li Feng bought 5 identical exercise books and 7 identical pencils and paid a total of 20.9 yuan. How much is this exercise book?
王亮和李峰一起去文具店购物，王亮买了5本练习本和4支铅笔，共付17.3元，李峰买了5本同样的练习本和7支同样的铅笔，共付20.9元，每本练习本多少元？
<gadget id="calculator">20.9 - 17.3</gadget>
<output>3.6</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

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

<gadget id="calculator">4 * 1.2</gadget>
<output>4.8</output>

<gadget id="calculator">17.3 - 4.8</gadget>
<output>12.5</output>

<gadget id="calculator">12.5 / 5</gadget>
<output>2.5</output>

<result>2.5</result>