The school cafeteria bought a batch of rice and used 45kg every day. After 18 days of use, there was still 90kg left. How many kilograms did this batch of rice originally have? (solution with equation)
学校食堂买回一批大米，每天用45kg，用了18天后，还剩90kg，这批大米原来有多少千克？(用方程解答)
<gadget id="calculator">45 * 18</gadget>
<output>810</output>

<gadget id="calculator">810 + 90</gadget>
<output>900</output>

<result>900</result>