The school bought 4 basketballs, which is half the number of volleyballs bought. The number of footballs bought was 7 times more than the total number of basketballs and volleyballs. How many footballs did they buy?
学校买来篮球4个，是买来排球数量的一半，买来的足球比篮球、排球总数的2倍多7个，买来多少个足球？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 4</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">24 + 7</gadget>
<output>31</output>

<result>31</result>