Analysis: Xiaohong is so old now that it will take 40 years for her grandfather to be born, so when Xiaohong was born, her grandfather was 40+x years old, which means her grandfather was 40+x years older than Xiaohong. So grandpa's age now is x+40+x years old. When Xiaohong was the same age as Grandpa, Grandpa was already 125 years old, that is, Grandpa's current age plus the part older than Xiaohong was 125 years old.

So:

x+40+x+40+x= 125

3x+80= 125

3x=45

x= 15

Grandpa's age: 15+ 15+45=70 (years old)

Grandpa is 70 years old now.