9× 6 = 54 years

Age gap between grandpa and grandson:

54-9 = 45 years old

When grandpa's age is four times that of his grandson, the grandson's age is:

45÷(4- 1)

=45÷3

= 15 (years old)

A: When Sun Zi 15 years old, his grandfather was four times his age.

Analysis:

When grandpa's age is four times that of his grandson, their age difference is exactly three times that of his grandson.