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The problem provided states:

- **Keliling sebuah persegi panjang adalah 40 cm.**
- **Jika ukuran panjangnya adalah 4 kali lebarnya, maka hitung luas persegi panjang tersebut!**

### Step-by-step solution:

1. **Perimeter of a rectangle formula**:
   \[
   P = 2 \times (panjang + lebar)
   \]
   We are given that the perimeter (P) is 40 cm:
   \[
   40 = 2 \times (panjang + lebar)
   \]
   Dividing by 2:
   \[
   20 = panjang + lebar
   \]

2. **Relationship between length and width**:
   The problem states that the length is 4 times the width:
   \[
   panjang = 4 \times lebar
   \]

3. **Substitute this into the perimeter equation**:
   \[
   20 = 4 \times lebar + lebar
   \]
   This simplifies to:
   \[
   20 = 5 \times lebar
   \]
   Solving for **lebar** (width):
   \[
   lebar = \frac{20}{5} = 4 \text{ cm}
   \]

4. **Calculate the length**:
   \[
   panjang = 4 \times lebar = 4 \times 4 = 16 \text{ cm}
   \]

5. **Calculate the area of the rectangle**:
   The formula for the area is:
   \[
   \text{Luas} = panjang \times lebar
   \]
   Substituting the values:
   \[
   \text{Luas} = 16 \times 4 = 64 \text{ cm}^2
   \]

### Final Answer:
The area of the rectangle is **64 cm²**.

Would you like further details or have any questions?

Here are 5 related questions for further practice:
1. If the perimeter was 60 cm with the same length-to-width ratio, what would the area be?
2. How does changing the ratio between length and width affect the perimeter?
3. What would the area be if the length was 3 times the width and the perimeter remained the same?
4. What is the diagonal of this rectangle?
5. If the width is doubled, how does that change the area of the rectangle?

**Tip**: Always check that your perimeter formula accurately reflects the relationship between length and width before solving for unknowns!

Keliling sebuah persegi panjang adalah 40 cm. Jika ukuran panjangnya adalah 4 kali lebarnya, maka hitung luas persegi panjang tersebut.

Learn how to calculate the area of a rectangle with a perimeter of 40 cm where the length is 4 times the width. This problem introduces key geometry concepts such as the perimeter and area of rectangles. Suitable for students in Grades 5-7.

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