Mom bought a watermelon, Xiaoming ate (1/5) of this watermelon, and mother ate (3/10) of this watermelon, what percentage of this watermelon is left?
妈妈买来一个西瓜，小明吃了这个西瓜的(1/5)，妈妈吃了这个西瓜的(3/10)，这个西瓜还剩几分之几？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (1/5) - (3/10)</gadget>
<output>1/2 = around 0.5</output>

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