Our school needs to paint the classroom. The classroom is 8 meters long, 7 meters wide, and 3.5 meters high. The area is 21 square meters after deducting the doors, windows, and blackboards. It is known that the paint fee is 4 yuan per square meter. How much does it cost to paint a classroom?
我们学校要粉刷教室，教室长8米，宽7米，高3.5米，扣除门窗、黑板的面积21平方米，已知每平方米需要4元涂料费．粉刷一个教室需要多少钱？
<gadget id="calculator">8 * 7</gadget>
<output>56</output>

<gadget id="calculator">8 * 3.5</gadget>
<output>28</output>

<gadget id="calculator">7 * 3.5</gadget>
<output>24.5</output>

<gadget id="calculator">28 + 24.5</gadget>
<output>52.5</output>

<gadget id="calculator">52.5 * 2</gadget>
<output>105</output>

<gadget id="calculator">105 - 21</gadget>
<output>84</output>

<gadget id="calculator">56 + 84</gadget>
<output>140</output>

<gadget id="calculator">140 * 4</gadget>
<output>560</output>

<result>560</result>