The school bought back 100 volleyballs and basketballs, and 120 basketballs and footballs. Given that the ratio of volleyballs to footballs is 3:4, how many volleyballs did the school buy back?
学校买回排球和篮球共100个，买回篮球和足球共120个．已知排球和足球个数的比是3：4，学校买回排球多少个？
<gadget id="calculator">120 - 100</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 1</gadget>
<output>20</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<result>60</result>