We have a regular tetrahedron with vertices ABCD.Length of each side being 1 unit.An insect starts at vertex A. At any vertex his probability of continuing on any of the three paths is 1/3 (including returning from the path from where it came). The insect starts at vertex A and travels 5 units.What is the probability that after travelling 5 untis he is still at vertex A?? Answer [20/81] .Tell me how to do it??