After drying, grass loses 80% of its original mass of water. A pile of hay weighs 500 kg, how much does it weigh before drying?
青草晒干后要失去相当于原来质量80%的水分．一堆干草重500千克，晒干前重多少千克？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">500 / (1/5)</gadget>
<output>2_500</output>

<result>2_500</result>