A barrel of oil, 40 kg is used up, and the amount used is less than the rest (1/5), how many grams is the original oil in the barrel.
一桶油，用去40千克，用去的比剩下的少(1/5)，这桶油原来有多少克．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">40 / (4/9)</gadget>
<output>90</output>

<gadget id="calculator">90 * 1_000</gadget>
<output>90_000</output>

<result>90_000</result>