(b) 7970000
For the first digit there are 8 choices (out of 10 digits) as 0 and 1 cannot be used. Since repetition can be done, the 2nd digit and the 3rd digit have 10 choices each. So, the first three digits can be filled in (8 × 10 × 10 – 3) ways (We need to exclude the numbers 555, 411 and 936 also from first three digits)
The last four digits of the telephone number can be filled in (10 × 10 × 10 × 10) ways.
∴ Total number of seven digit phone numbers = (8 × 10 × 10 – 3) × 10 × 10 × 10 × 10 = 7970000.
