assuming is square root of 55:
You can solve the square root and roound to the nearest number:
√55 = 7.41
Since the number after the decimal point is 4, the square root is closest to 7 than to 5.
You can also campare √55 with closest integers that are perfect square:
below 55, the closest perfect square is 49 = 7^2
above 55, the closest perfect square is 64 =8^2
55-49 = 6
64-55 = 10
Since 49 is closest to 55, it's square root is the nearest. (7)