The elder brother has 20 pens and the younger sister has 10 pens. After the younger sister gave the older brother a few pens, the older brother had four times as many pens as the younger sister.
哥哥有20支笔，妹妹有10支笔．妹妹给哥哥多少支笔后，哥哥的笔是妹妹的4倍．
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">30 / 5</gadget>
<output>6</output>

<gadget id="calculator">10 - 6</gadget>
<output>4</output>

<result>4</result>