Mom bought a piece of cake, Xiaodong ate (2/5) of this cake, mom ate (1/5) of this cake, and dad ate the rest. What fraction of the cake did dad eat?
妈妈买回一块蛋糕，小东吃了这块蛋糕的(2/5)，妈妈吃了这块蛋糕的(1/5)，剩下的爸爸吃．爸爸吃了这块蛋糕的几分之几？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">1 - (2/5) - (1/5)</gadget>
<output>2/5 = around 0.4</output>

<result>2/5 = around 0.4</result>