There are 91 balls and footballs in total. The ratio of balls to footballs is 2:5. How many fewer balls are there than footballs?
皮球和足球一共有91个，皮球和足球的比是2：5，皮球比足球少多少个？
<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<gadget id="calculator">91 / 7</gadget>
<output>13</output>

<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<result>39</result>