The famous Russian mathematician Lomonosov borrowed the book "Principles of Mathematics" from his neighbor, and the neighbor said to her: "If you help me chop firewood for 10 days, I will give you the book and give you another 20 rubles." Only 7 days of firewood were chopped. After the neighbor gave him the book, he paid another 5 rubles. How much is the price of the book "Principles of Mathematics" in rubles?
俄国著名数学家罗蒙诺索夫向邻居借《数学原理》一书，邻居对她说："你帮我劈10天柴，我就把书送给你，另给你20卢布．"结果他只劈了7天柴．邻居把书送给他后，另外付了5个卢布．《数学原理》这本书的价格是多少卢布？
<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">10 - 7</gadget>
<output>3</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 7</gadget>
<output>35</output>

<gadget id="calculator">35 - 5</gadget>
<output>30</output>

<result>30</result>