There are three empty measuring cups A, B, and C. The capacity of cup A is (1/5) liter and the capacity of cup B is (1/4) liter. If both cups A and B are filled with water and poured into cup C together, then the water in cup C will account for (9/10). What is the capacity of the C cup in liters?
有甲、乙、丙三个空量杯．甲杯的容量是(1/5)升，乙杯的容量是(1/4)升．如果将甲、乙两杯都装满水，一起倒入丙杯，这时，丙杯中的水约占(9/10)．丙杯的容量是多少升？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) + (1/4)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(9/20) / (9/10)</gadget>
<output>1/2 = around 0.5</output>

<result>1/2 = around 0.5</result>