The school library bought some "Elementary School Mathematics Olympiad Morning Train", and after lent out (4/7), there are still 9 copies left. How many copies of "Primary School Mathematics Olympiad Morning Train" have been purchased by the school library?
学校图书馆购进一些《小学数学奥利匹克早班车》，借出(4/7)后，还剩9本．学校图书馆共购进《小学数学奥利匹克早班车》多少本？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (4/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">9 / (3/7)</gadget>
<output>21</output>

<result>21</result>