Three books of one kind cost 21 yuan in total. Teacher Zhu buys this kind of book for 350 yuan. How many books can he buy in total?
一种书3本共21元，朱老师用350元买这种书，一共可买多少本？
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">350 / 7</gadget>
<output>50</output>

<result>50</result>