A rope (9/10) m long is used up (1/5) of the full length once, and (3/10) of the full length is used up for the second time, what fraction of the full length is left?
一根绳子长(9/10)m经一次用去全长的(1/5)，第二次用去全长的(3/10)，还剩全长的几分之几？
<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>