A piece of iron wire, the full length (2/5) is used for the first time, and 21 meters is used for the second time. The ratio of the remaining length to the used length is 1:3. How long is this iron wire?
一根铁丝，第一次用去全长的(2/5)，第二次用去21米，剩下的与用去的长度比是1：3，这根铁丝原来有多长？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(3/4) - (2/5)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">21 / (7/20)</gadget>
<output>60</output>

<result>60</result>