There is a batch of children's tops in the store, 30% of them are sold in the morning, and 40 pieces are sold in the afternoon. At this time, half of them are left. How many pieces of this batch of children's tops are there?
商店有一批儿童上衣，上午售出30%，下午售出了40件，这时正好剩下一半，这批儿童上衣有多少件？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">40 / (1/5)</gadget>
<output>200</output>

<result>200</result>