A kind of concrete is composed of cement, yellow sand and gravel, among which yellow sand accounts for (1/2), gravel accounts for (1/5), how much cement accounts for
一种混凝土由水泥、黄沙和石子组成，其中黄沙占(1/2)，石子占(1/5)，水泥占多少
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<result>3/10 = around 0.3</result>