The school cafeteria originally had 100 kilograms of rice. After eating 40 percent, how many kilograms were bought, and the rice in the cafeteria was 90 percent of the original.
学校食堂原有大米100千克，吃掉40%后，又买进多少千克，这时食堂的大米是原来的90%．
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">100 * (9/10)</gadget>
<output>90</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">100 * (3/5)</gadget>
<output>60</output>

<gadget id="calculator">90 - 60</gadget>
<output>30</output>

<result>30</result>