For a bucket of water, pour out 1/4 of the total amount for the first time, pour out 12L for the second time, and there are 9L left. How many liters of water are poured out for the first time?
一桶水，第一次倒出总数的(1/4)，第二次倒出12L，还剩下9L，第一次倒出的水有多少升？
<gadget id="calculator">12 + 9</gadget>
<output>21</output>

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

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">21 / (3/4)</gadget>
<output>28</output>

<gadget id="calculator">28 * (1/4)</gadget>
<output>7</output>

<result>7</result>