A bottle of oil weighs (9/10) kilograms, use up (3/10) of it on the first day, and use up (1/5) of it in the second day, what fraction of it is left useless?
一瓶油重(9/10)千克，第一天用去它的(3/10)，第二天用去它的(1/5)，还剩它的几分之几没用？
<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

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

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

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