25% of a barrel of oil is used, and there are 21 kilograms left. How many kilograms of this barrel of oil are there?
一桶油，用去25%，还剩21千克，这桶油原有多少千克？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">21 / (3/4)</gadget>
<output>28</output>

<result>28</result>