The sports group has 13 volleyballs and 15 footballs. These balls are less than the total number of school balls (3/7). How many are the total number of school balls?
体育组有排球13个，足球15个，这些球比学校球类总数少(3/7)，学校球类总数是多少个？
<gadget id="calculator">13 + 15</gadget>
<output>28</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 - (3/7)</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">28 / (4/7)</gadget>
<output>49</output>

<result>49</result>