6. What is the number of dec-strings d,..., dn of length n > 4 such that did2 # 00 or d3d4 # 11. 7. A dec-string d1, ..., dn is bad if di = di+1 or di +di+1 =9 for at least one E {1,...,n-1} and it is good otherwise. What is the number of good dec-strings of length n? 8. A dec-string d1, ..., dn is 2-bad if, di = d; or d +d; = 9 for some i