The number of people in class A (4/5) is equivalent to the number of people in class B. 75% of the total number of people in the two classes is 81 people. How many people are there in class B?
甲班人数的(4/5)相当于乙班的人数，两班总人数的75%是81人，乙班有多少人．
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">81 / (3/4)</gadget>
<output>108</output>

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

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">108 * (4/9)</gadget>
<output>48</output>

<result>48</result>