Class 51 students were divided into three groups to participate in the cleaning. The number of the first and second groups accounts for (2/5) of the whole class, and the number of the second and third groups accounts for (9/10) of the whole class. What percentage of the whole class is the number of the second group?
五1班同学被分成三个小组去参加大扫除．第一、二两组的人数占全班人数的(2/5)，第二、三两组的人数占全班人数的(9/10)．第二小组的人数占全班人数的几分之几？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(9/10) - 1</gadget>
<output>-1/10 = around -0.1</output>

<gadget id="calculator">(2/5) + (-1/10)</gadget>
<output>3/10 = around 0.3</output>

<result>3/10 = around 0.3</result>