Mr Fong bought some plastic chairs. 20% of them were blue and the rest were yellow. He sold 3/4 of the blue chairs and 75% of the yellow chairs. There were 150 chairs left. How many chairs did Mr Fong sell?

Let's solve the problem step by step.

### Step 1: Define the total number of chairs
Let the total number of chairs be \( x \).

### Step 2: Calculate the number of blue and yellow chairs
- **Blue chairs**: 20% of \( x \) = \( 0.2x \)
- **Yellow chairs**: The rest are yellow, so 80% of \( x \) = \( 0.8x \)

### Step 3: Determine how many chairs were sold
- **Blue chairs sold**: \( \frac{3}{4} \) of the blue chairs were sold, so \( \frac{3}{4} \times 0.2x = 0.15x \)
- **Yellow chairs sold**: 75% of the yellow chairs were sold, so \( 0.75 \times 0.8x = 0.6x \)

### Step 4: Calculate the number of chairs left
The total number of chairs left after selling is:
\[
\text{Chairs left} = \text{Blue chairs left} + \text{Yellow chairs left}
\]
\[
= (0.2x - 0.15x) + (0.8x - 0.6x) = 0.05x + 0.2x = 0.25x
\]
According to the problem, there were 150 chairs left:
\[
0.25x = 150
\]

### Step 5: Solve for \( x \)
\[
x = \frac{150}{0.25} = 600
\]
So, the total number of chairs Mr. Fong originally had is 600.

### Step 6: Calculate the number of chairs sold
The total number of chairs sold is:
\[
\text{Chairs sold} = 0.15x + 0.6x = 0.75x
\]
Substitute \( x = 600 \):
\[
\text{Chairs sold} = 0.75 \times 600 = 450
\]

### Final Answer:
Mr. Fong sold **450** chairs.

Would you like more details or have any questions about this solution?  

Here are five questions related to the concepts used:

1. What percentage of the yellow chairs were left after the sale?
2. How would the number of chairs left change if Mr. Fong sold only 50% of the yellow chairs?
3. If Mr. Fong had started with 800 chairs, how many would he have sold?
4. What fraction of the total chairs sold were blue?
5. How many chairs would be left if 80% of the blue chairs and 60% of the yellow chairs were sold?

**Tip:** When dealing with percentages in word problems, converting them to decimals can simplify calculations significantly.

Learn how to solve Mr. Fong's problem involving the distribution and sale of blue and yellow chairs. Understand the application of percentages and fractions in determining the number of chairs sold. Suitable for students in Grades 7-9.

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