A barrel of oil, the first time used the full barrel of oil (1/5), the second time used the full barrel of oil (1/4), and the third time used less than the sum of the previous two (1/5 ), what fraction of the full barrel of oil was used for the third time?
一桶油，第一次用了全桶油的(1/5)，第二次用了全桶油的(1/4)，第三次用的比前两次的和少(1/5)，第三次用了全桶油的几分之几？
<gadget id="calculator">9 / 20</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(9/20) * (3/5)</gadget>
<output>27/100 = around 0.27</output>

<result>27/100 = around 0.27</result>