Use two cups to hold water, and pour (1/5) of cup A into cup B. At this time, the two cups have the same amount of water. Originally, cup B contained 15 liters of water, and how many liters of water did cup A originally contain.
用两个杯子盛水，把甲杯的(1/5)倒入乙杯，这时两杯水就一样多，原来乙杯有水15升，甲杯原来有水多少升．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">15 / (3/5)</gadget>
<output>25</output>

<result>25</result>