In a two-layer bookshelf, the number of books on the upper layer is three times that of the lower layer. If 10 books are taken from both layers, the number of books on the upper layer is four times that of the lower layer. It turns out how many books are there on the upper floor.
一个两层书架，上层书的本数是下层的3倍，如果从两层都拿走10本书，这时上层书的本数是下层的4倍．原来上层有书多少本．
<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">30 / 1</gadget>
<output>30</output>

<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<result>90</result>