10% of a colored ribbon is used for the first time, and 3/10 of it is used for the second time, and 4 meters are used for the two times. How many meters is the total length of this colored ribbon?
一条彩带第一次用去它的10%，第二次用去它的(3/10)，两次共用去4米，这根彩带全长多少米？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (3/10)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 / (2/5)</gadget>
<output>10</output>

<result>10</result>