Let Peter's age be represente by the variable "x".

Since Chris is four times the age of Peter, Chris' age can be represented by "4x".

Sara is 6 years less than Chris, therefore "4x - 6" can represent Sara's age.

Together their ages total 66 years.

`(x) + (4x) + (4x - 6) = 66`

`9x - 6 = 66`

`9x = 72`

`x = 8`

This means that Peter is 8 years old, Chris is 32 years old, and Sara is 6 years younger, or 26 years old.