A batch of wood has a total of 30 square meters. Half of the total amount is used up on the first day, and 1/5 of the total amount is used up on the second day. How many square meters are left in this batch of wood?
一批木材，一共有30方，第一天先用去了总数的一半，第二天又用去了总数的(1/5)，这批木材还剩多少方．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">30 * (3/10)</gadget>
<output>9</output>

<result>9</result>