There are not enough teaching materials for the sixth grade in a certain school. For the time being, every two students use one Chinese textbook, every three students use one math textbook, and every four students use one foreign language textbook. The whole grade shares 91 textbooks. How many people are there in the whole grade?
某校六年级的教材不够，暂时每两人用一本语文课本，每三人用一本数学课本，每四人用一本外语课本，全级共用了91本课本，全级有多少人．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/2) + (1/3) + (1/4)</gadget>
<output>13/12 = around 1.083333</output>

<gadget id="calculator">91 / (13/12)</gadget>
<output>84</output>

<result>84</result>