Answer:
The number that does not belong with the other three is √50.
Step-by-step explanation:
Consider the provided numbers.
Rational number: A number  
is said to be rational, if it is in the form  
of p/q. Where p and q are integer and  
denominator is not equal to 0. The decimal expansion of rational numbers may terminate or become periodic.
Irrational  
number: A number is irrational if it cannot  
be expressed be expressed by dividing two  
integers. The decimal expansion of  
Irrational numbers are neither terminate nor  
periodic.
Now consider the number.
50.1 repeating 1 is a rational number as the decimal expansion is periodic.
Negative 50 over 2 can be written as: -50/2
-50/2 is a rational number as it is in the form of p/q.
Negative 50.1 can be written as: -501/10
Thus, -501/10 is a rational number as it is in the form of p/q.
√50 = 7.071067...
The number √50 is not a rational number as the decimal expansion of  
number is neither terminate nor  
periodic.
Hence, the number √50 is an irrational number.
The number that does not belong with the other three is √50.