25
Let w represent the weight of the animal in pounds, and d the age in days. The problem statement tells us ...
... w = 75 +2d
We want to find d when w > 125.
... 75 +2d > 125 . . . . . substitute the above expression for w
... 2d > 50 . . . . . . . . . subtract 75
... d > 25 . . . . . . . . . . . divide by 2
The animal will weigh more than 125 pounds when it is more than 25 days old.