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.