Subtract a natural number from the numerator and denominator of (21/81) and reduce the new fraction to get (4/19). What is this number?
将(21/81)的分子、分母同时减去一个自然数，新分数约简得后得(4/19)，这个数是多少．
<gadget id="calculator">75 / 15</gadget>
<output>5</output>

<result>5</result>