Then the length of the stick is 5 cm.
So fill in 5.
With the help of the number axis, the age difference between Xiaohong and grandpa is regarded as a stick AB.
Like when grandpa is older than Xiaohong, when point A moves to point B,
At this point, the number corresponding to point B is -40.
When Xiaohong is older than grandpa, she thinks that when point B moves to point A,
At this time, the number corresponding to point A is 125.
∴ Grandpa is older than Xiaohong [125-(-40)]÷3=55
Grandpa's age is 125-55 = 70.
So fill 70.