Explanation:
we know the square numbers of the basic integer numbers :
1² = 1
2² = 4
3² = 9
4² = 16
5² = 25
6² = 36
7² = 49
8² = 64
9² = 81
10² = 100
so, we see when we square the inequality, the closest numbers are
81 < 91 < 100
and therefore
9 < sqrt(91) < 10