A barrel of oil weighs 50 kilograms. The first time (3/10) is used, and the rest (1/7) is used the second time. How many kilograms of oil are left in the barrel?
一桶油重50千克，第一次用去(3/10)，第二次用去剩下的(1/7)，这桶油还剩下多少千克？
<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(7/10) * (1/7)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (3/10) - (1/10)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">50 * (3/5)</gadget>
<output>30</output>

<result>30</result>