batch of fabrics can be made into 20 pieces if they are all used as tops, and 30 pieces if they are all used as trousers; after making 3 sets, the rest can be used as tops, how many tops can be made?
一批布料，如果全部用做上衣则可做20件，如全部用做裤子则可做30条；现配套做了3套后，剩下的再做上衣，还可以做多少件上衣？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) / (1/20)</gadget>
<output>15</output>

<result>15</result>