To pave the living room with square bricks with a side length of 20 cm, 900 pieces are needed. If you use square bricks with a side length of 30 cm instead, how many bricks are needed? (use proportional knowledge and other methods to answer)
用边长20厘米的方砖给客厅铺地，需要900块．如果改用边长30厘米的方砖，需要多少块？(用比例知识和其他方法解答)
<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<gadget id="calculator">30 * 30</gadget>
<output>900</output>

<gadget id="calculator">900 / 900</gadget>
<output>1</output>

<gadget id="calculator">400 * 1</gadget>
<output>400</output>

<result>400</result>