For a barrel of oil, 1/5 of the total amount was used on the first day, and the rest (3/4) was used on the second day, leaving 15 kilograms. How many kilograms is this barrel of oil originally?
一桶油，第一天用了总量的(1/5)，第二天用了剩下的(3/4)，还剩15千克．这桶油原有多少千克？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">15 / (1/4) / (4/5)</gadget>
<output>75</output>

<result>75</result>