There are 450 books on the three shelves A, B, and C. If you take out 60 books from A and put them in B, then take out 20 books from B and put them in C, and finally take out 30 books from C and put them in A, then the bibliographies of the books on the three shelves are equal. How many books are there on shelf A?
甲、乙、丙三个书架上共有书450本．若从甲拿出60本放入乙中，再从乙拿出20本放入丙中，最后再从丙拿出30本放入甲中，这时三个书架上书的书目相等．甲书架上原有书多少本．
<gadget id="calculator">450 / 3</gadget>
<output>150</output>

<gadget id="calculator">150 - 30</gadget>
<output>120</output>

<gadget id="calculator">120 + 60</gadget>
<output>180</output>

<result>180</result>