Grandma Zhang raises 72 white rabbits, 20% less than black rabbits. How many black rabbits does Grandma Zhang raise?
张奶奶养白兔72只，比黑兔少20%，张奶奶养黑兔多少只？
<gadget id="calculator">20 / 100</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">72 / (4/5)</gadget>
<output>90</output>

<result>90</result>