Let the larger number be x
Let the smaller number be y
We were told that the two numbers have a sum of 139. This means that
x + y = 139
x = 139 - y
Also, we were told that the difference of the numbers is 31. This means that
x - y = 31
Substituting x = 139 - y into x - y = 31, we have
139 - y - y = 31
- y - y = 31 - 139
- 2y = - 108
y = - 108/-2
y = 54
x = 139 - y = 139 - 54
x = 85
Thus, one of the numbers is 85