The two brothers share 15 baseball cards; if the elder brother sends 3 cards to the younger brother, he has twice as many baseball cards as the younger brother. Try to figure out how many baseball cards brother Ben has.
兄弟两人共有棒球卡15张；若哥哥送了3张给弟弟后，他所有的棒球卡是弟弟的2倍．试算出哥哥本有多少张棒球卡．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

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

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">15 - 2</gadget>
<output>13</output>

<result>13</result>