I did it by trial and error.
7^2 is 49.
4 + 9 = 13
81
Every square of a 4 digit number has more than four digits.
81...........
81
121
81
49
Every square of a 4 digit number has more than four digits.
81...........
81
1
121
81
The largest 3-digit number - not itself a perfect square - is 999. Calculate its square root, round the answer down, then square the answer again.
algorithm is a way to solve your problem
The number is 81.
The square root of 55 is an irrational number, meaning it never ends and doesn't have a repeating digit pattern, so it has an infinite number of digits.