There are 3 red balls of the same size and material in the pocket, 4 white balls and 5 black balls. If the probability of drawing a red ball is (1/2), how many red balls should be put in the pocket? ball.
口袋中有大小、材质相同的红球3个，白球4个和黑球5个，要想使从中摸出一个红球的可能性是(1/2)，应该再往口袋中放多少个红球．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">9 / (1/2)</gadget>
<output>18</output>

<gadget id="calculator">18 - 9 - 3</gadget>
<output>6</output>

<result>6</result>