S = D + 6. ("Sally is 6 years older than Donald") (D + 6) + D = 18 ("The 2 children have a total age of 18 years") =========================> 2D + 6 = 18 ====> 2D = 18-6 = 12 ====> D == 6. Answer. Donald is 6 years old, Sally is 12.
x, Donald's age x+6, Sally's age