The experimental school used 405 tons of water in May, 10% less than in April. How many tons of water was used in April?
实验学校五月份用水405吨，比四月份节约了10%，四月份用水多少吨？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">405 / (9/10)</gadget>
<output>450</output>

<result>450</result>