After adding the same number to the numerator and denominator of the fraction (27/11), the ratio of the numerator to the denominator is 21:13. What is the number?
分数(27/11)的分子和分母都加上同一个数后，分子和分母的比是21：13，求这个数是多少？
<gadget id="calculator">21 / 13</gadget>
<output>21/13 = around 1.615385</output>

<gadget id="calculator">(21/13) * 11</gadget>
<output>231/13 = around 17.769231</output>

<gadget id="calculator">27 - (231/13)</gadget>
<output>120/13 = around 9.230769</output>

<gadget id="calculator">(21/13) - 1</gadget>
<output>8/13 = around 0.615385</output>

<gadget id="calculator">(120/13) / (8/13)</gadget>
<output>15</output>

<result>15</result>