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The problem is about forming an 8-digit phone number using digits 0 through 9, under certain conditions. Let's break down and solve each part:

### Problem Analysis:
1. The phone number must be 8 digits long.
2. The first digit **cannot** be 0 (it should be 1 to 9).

### Solutions:
Let's calculate the number of phone numbers under each scenario:

#### a. **Angka-angka boleh berulang** (Digits can repeat)
- For the first digit, we have 9 options (1-9).
- For each of the remaining 7 digits, all 10 digits (0-9) can be used.

Thus, the number of phone numbers is:
\[
9 \times 10^7 = 90,000,000
\]

#### b. **Tidak boleh ada angka yang diulang** (No digit repetition)
- For the first digit, we have 9 options (1-9).
- For the second digit, we have 9 options (0-9, excluding the first digit).
- For the third digit, 8 options remain, and so on.

So, the number of phone numbers is:
\[
9 \times 9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 = 9 \times 9! / (9 - 7)!
\]
Calculating:
\[
9 \times 9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 = 9 \times 181,440 = 1,632,960
\]

#### c. **Hanya angka pertama yang tidak boleh diulang** (Only the first digit cannot repeat)
- For the first digit, we have 9 options (1-9).
- For each of the remaining 7 digits, any of the 10 digits can be used, including the first digit.

Thus, the number of phone numbers is:
\[
9 \times 10^7 = 90,000,000
\]

### Summary:
- **(a)**: 90,000,000 phone numbers
- **(b)**: 1,632,960 phone numbers
- **(c)**: 90,000,000 phone numbers

Would you like more details on any of these solutions or have any specific questions? Here are 5 related questions for further exploration:

1. How would the calculation change if the phone number length were increased to 9 digits?
2. What would happen if the first digit also allowed 0?
3. Can we derive a general formula for cases with varying digit restrictions?
4. What is the probability of selecting a number without repetition if chosen randomly?
5. How many unique phone numbers can be generated if only certain digits are allowed?

**Tip**: When counting possibilities, always account for restrictions first (e.g., "no repetition") before applying combinatorics.

Akan disusun nomor telepon rumah yang terdiri atas 8 angka, dengan ketentuan angka pertama tidak boleh angka 0. Tentukan banyaknya nomor telepon yang dapat dibuat dari angka-angka 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, jika: a. angka-angka boleh berulang b. tidak boleh ada angka yang diulang c. hanya angka pertama yang tidak boleh diulang.

This problem explores how to calculate the number of possible 8-digit phone numbers using digits 0-9, with the first digit restricted. Solutions for cases with repetition, without repetition, and with partial repetition restrictions are provided. Suitable for high school math students.

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