It is known that A pool is filled in 6 hours and B is filled in 4 hours. After 2 hours of water filling at the same time, the pool can still hold 3.5 cubic meters of water. What is the capacity of the pool?
已知某水池，甲6小时注满，乙4小时注满，同时注水2小时后，水池还能装3.5立方米的水，水池容量多少？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/6) + (1/4)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">2 * (5/12)</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">3.5 / (1/6)</gadget>
<output>21</output>

<result>21</result>