There is a batch of books that sold 400 copies on the first day, 1/3 of the total on the second day, and 30% of the total on the third day, leaving 150 copies left. How many books are there in total?
有一批书，第一天卖出400本，第二天卖出占总数的(1/3)，第三天卖出总数的30%，还剩下150本．这批书一共有多少本？
<gadget id="calculator">400 + 150</gadget>
<output>550</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (1/3) - (3/10)</gadget>
<output>11/30 = around 0.366667</output>

<gadget id="calculator">550 / (11/30)</gadget>
<output>1_500</output>

<result>1_500</result>