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In the US, telephone numbers are 7-digit long. While in general they have some restrictions, for example, they cannot start w

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Answer #1

1)start or end with two identical digits =N(here we need to select digits for 6 places only as 1st place and 7th place are same)

=106 =1000000

2)total type of 5 digit conecutive substring =6 (starting with 0 to starting with 5)

hence number of such strings =N(3 places in a 7 string and choose other 2 digits)=10*10*3*6=1800

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