Answer:
For the seven-digit telephone number in the form ABC-DEFG:
- The digit 'A' can be any number between 1 and 8, giving us 8 choices.
- The digits 'B' and 'C' can each be any number between 3 and 5, which gives us 3 choices for each of these digits.
- The digits 'D', 'E', 'F', and 'G' have no restrictions, meaning each can be any digit from 0 to 9, giving us 10 choices for each of these digits.
To find the total number of possible seven-digit phone numbers, we can multiply the number of choices for each digit:
\[ 8 (choices for A) \times 3 (choices for B) \times 3 (choices for C) \times 10 (choices for D) \times 10 (choices for E) \times 10 (choices for F) \times 10 (choices for G) \]
\[ = 8 \times 3 \times 3 \times 10 \times 10 \times 10 \times 10 \]
\[ = 8 \times 9 \times 10^4 \]
\[ = 72 \times 10,000 \]
\[ = 720,000 \]
So, 720,000 different seven-digit phone numbers are possible with these restrictions.