Dad =4 Xiao Gan

Mom = Dad-1=4 Xiao Gan-1

So 4 Xiao Gan +4 Xiao Gan-1+ Xiao Gan =80 years old.

So the formula of Xiao Gan's age is (80+ 1) divided by (4+ 1+4)=9.

In other words, Xiao Gan is 9 years old.

Please accept it if you are satisfied.