Video solusi : Call a 7-digit telephone number a1a2a3- a4a5a6a7 memorable if the prefix sequence a1a2a3 is exactly the same as either of the sequence a4a5a6 or a5a6a7 (possibly both). Assumming that each ai can be any of the ten decimal digits 0, 1, 2, ..., 9, find the possible number of different memorable telephone numbers.