A rope, 20% of the total length is used for the first time, 30% of the total length is used for the second time, and 15 meters are used for the two times. How many meters is this rope?
一根绳，第一次用去全长的20%，第二次用去全长的30%，两次一共用去了15米，这根绳原来有多少米？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/5) + (3/10)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">15 / (1/2)</gadget>
<output>30</output>

<result>30</result>