The students line up to do exercises, the naughty ones are ranked 5th from the left, the 7th from the right; the 4th from the front, and the 6th from the back. Ask how many people are doing exercises.
同学们排队做操，从左数淘气排在第5个，从右数排在第7个；从前数淘气排在第4个，从后数排在第6个．问有多少人在做操．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">5 + 6</gadget>
<output>11</output>

<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">11 * 9</gadget>
<output>99</output>

<result>99</result>