Rationalization method is to be used while solving sums like these
Rationalization factor for :
1) 1/√a ------> √a
2) a + √b -------> a - √b
3) a - √b ---------> a + √b
4) √a + √b ----------> √a - √b
5) √a - √b ------------> √a + √b

642 / √932
As seen in above Rationalization Methods we can use 1^st rule to solve the problem
Thus 642 / √932 is multiplied the rationalizing factor i.e. √932
To balance the extra added rationlizing factor we multiply the given ratio by √932 / √932
Thus the result of (642 / √932) * (√932 / √932) = (642 * √932) / 932
Thus 642 / √932 = (642 * √932) / 932 = 21.029409204781
Thus 642 / √932 = (642 * √932) / 932 = 21(round off to nearest integer)