There is a barrel of oil, (1/4) of the total is taken out for the first time, (2/5) of the total number is taken out for the second time, and 7.5 kg more is taken out for the second time than the first time. How many kilograms were taken out for the first time?
有一桶油，第一次取出总数的(1/4)，第二次取出总数的(2/5)，第二次比第一次多取出7.5千克．第一次取出多少千克？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(2/5) - (1/4)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">7.5 / (3/20)</gadget>
<output>50</output>

<gadget id="calculator">50 * (1/4)</gadget>
<output>25/2 = around 12.5</output>

<result>25/2 = around 12.5</result>