There are three balls of red, yellow and blue in the bag. If one ball is chosen from among them, the probability of picking a red ball is 20%. Now add 3 yellow balls to the bag, then the probability of taking out the yellow balls is 25%; take away 8 blue balls, then the probability of taking out the blue balls is 50%. How many blue balls are there in the bag?
袋中有红、黄、蓝3种颜色的球若干个．从中取任取一个球，取出红球的可能性是20%．现往袋中加入3个黄球，这时取出黄球的可能性是25%；再拿走8个蓝球，这时取出蓝球的可能性是50%．袋中原有蓝球多少个．
<gadget id="calculator">5 - 2.5</gadget>
<output>2.5</output>

<gadget id="calculator">0.75 + 2.5</gadget>
<output>3.25</output>

<gadget id="calculator">1 - 0.2 - 0.25 - 0.5</gadget>
<output>0.05</output>

<gadget id="calculator">3.25 / 0.05</gadget>
<output>65</output>

<gadget id="calculator">3 - 8</gadget>
<output>-5</output>

<gadget id="calculator">65 + (-5)</gadget>
<output>60</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">60 * (1/2)</gadget>
<output>30</output>

<gadget id="calculator">30 + 8</gadget>
<output>38</output>

<result>38</result>