There is a batch of steel in the warehouse, 20% of the total is used for the first time, and (1/2) of the total is used for the second time. At this time, there are 4.5 tons left in the warehouse. How many tons are there in this batch of steel?
仓库里有一批钢材，第一次用去总数的20%，第二次用去总数的(1/2)．这时仓库里还剩下4.5吨，这批钢材一共有多少吨？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">4.5 / (3/10)</gadget>
<output>15</output>

<result>15</result>