No matter in a few years, the age difference between Dad and Tong Tong will remain unchanged, 34-7=27.

When my father's age is four times that of Tong Tong, the age difference is just three times that of Tong Tong, and 27/3=9.

At that time, Tong Tong was nine years old. Subtracting the current age is the answer, 9-7=2.