certain school held a model airplane competition, with a number of first, second, and third prizes. Those who won the first and second prizes accounted for 9/10 of the total number of winners, and those who won the second and third prizes accounted for 9/10 of the total number of winners. (2/5), the number of second prize winners in the total number of winners.
某校举办一次航模比赛，设一、二、三等奖若干名，竞赛结果获一、二等奖的占获奖总人数的(9/10)，获二、三等奖的占获奖总人数的(2/5)，获二等奖的占获奖总人数的多少．
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) - 1</gadget>
<output>-3/5 = around -0.6</output>

<gadget id="calculator">(9/10) + (-3/5)</gadget>
<output>3/10 = around 0.3</output>

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