The school bought 1620 new books, which is 20 books less than twice the original stock. How many books did the school have?
学校买来新图书1620本，比原来存书的2倍少20本，学校原来有图书多少本？
<gadget id="calculator">1_620 + 20</gadget>
<output>1_640</output>

<gadget id="calculator">1_640 / 2</gadget>
<output>820</output>

<result>820</result>