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We are given a demand function \( Q_d = 20 - P \), where \( P \) is the price. Additionally, the problem provides two pieces of information about supply at different prices:

1. When the price is Rp3.00, the quantity supplied \( Q_s = 8 \) units.
2. When the price is Rp4.00, the quantity supplied \( Q_s = 10 \) units.

The task is to find the equilibrium price. At equilibrium, the quantity demanded \( Q_d \) equals the quantity supplied \( Q_s \).

---

### Step 1: Find the supply function \( Q_s = aP + b \)
We need to assume a linear form for the supply function:

\[
Q_s = aP + b
\]

Using the two points (P, Q_s) given:
- At \( P = 3 \), \( Q_s = 8 \).
- At \( P = 4 \), \( Q_s = 10 \).

These two equations help us determine \( a \) and \( b \):

1. \( 8 = 3a + b \)
2. \( 10 = 4a + b \)

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### Step 2: Solve the system of equations for \( a \) and \( b \)
Subtract the first equation from the second:

\[
(10 - 8) = (4a - 3a) \implies 2 = a
\]

Now substitute \( a = 2 \) into the first equation:

\[
8 = 3(2) + b \implies 8 = 6 + b \implies b = 2
\]

Thus, the supply function is:

\[
Q_s = 2P + 2
\]

---

### Step 3: Find the equilibrium price
At equilibrium, \( Q_d = Q_s \). Substituting the functions:

\[
20 - P = 2P + 2
\]

Solve for \( P \):

\[
20 - 2 = 2P + P \implies 18 = 3P \implies P = 6
\]

---

### Final Answer
The equilibrium price is **Rp6.00**.

Thus, the correct answer is:

**B. Rp6,00**

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Do you have any further questions or need more details?

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**Related Questions:**
1. How is the demand curve affected if the intercept in the demand function changes?
2. What happens if the supply curve shifts upward due to increased production costs?
3. How do we find equilibrium quantity along with equilibrium price?
4. What is the impact of price ceilings on equilibrium price and quantity?
5. How can this problem be represented graphically using supply and demand curves?

**Tip:** In linear functions, two points are enough to determine the equation of a line, allowing you to solve for unknown constants easily.

Diketahui fungsi permintaan suatu barang Qd = 20 - P. Pada saat harga Rp3,00 penawaran barang sebanyak 8 unit, ketika harga Rp4,00 penawaran 10 unit. Besarnya harga keseimbangan adalah….

Learn how to find the equilibrium price using the demand function Qd = 20 - P and supply conditions at different prices. This problem involves solving a system of linear equations and is suitable for high school students.

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