For a piece of iron wire, use up the full length (1/5) for the first time, and use up 15 meters for the second time. At this time, the ratio of the used length to the remaining length is exactly 4:1. How long is this wire?
一根铁丝，第一次用去全长的(1/5)，第二次用去15米，这时用去的与剩下的长度比正好是4：1．这根铁丝长多少米？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

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

<gadget id="calculator">(4/5) - (1/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">15 / (3/5)</gadget>
<output>25</output>

<result>25</result>