How many bricks, (each measuring `20cm xx 16cm xx 8cm`), will be required to build a wall 30m long, 30cm thick and 5m high, with a provision of 2 doors, each measuring `2.5 m xx 1.2m`. It is given that one-ninth of the wall is occupied by the cement and the sand mixture.

Answer

To solve the problem step by step, let's break it down:

### Step 1: Convert all measurements to meters
- The dimensions of the bricks are given as `20 cm x 16 cm x 8 cm`.
- Convert these measurements to meters:
- Length: 20 cm = 0.2 m
- Width: 16 cm = 0.16 m
- Height: 8 cm = 0.08 m

### Step 2: Calculate the total area of the wall
- The wall dimensions are:
- Length = 30 m
- Thickness = 30 cm = 0.3 m
- Height = 5 m
- The formula for the surface area of the wall is:
\[ \text{Area} = 2 \times (\text{Length} \times \text{Height} + \text{Height} \times \text{Thickness} + \text{Thickness} \times \text{Length}) \]
- Substituting the values:
\[ \text{Area} = 2 \times (30 \times 5 + 5 \times 0.3 + 0.3 \times 30) \]
\[ = 2 \times (150 + 1.5 + 9) = 2 \times 160.5 = 321 \text{ m}^2 \]

### Step 3: Calculate the area of the doors
- Each door measures `2.5 m x 1.2 m`.
- Area of one door:
\[ \text{Area}_{\text{door}} = 2.5 \times 1.2 = 3 \text{ m}^2 \]
- Since there are two doors:
\[ \text{Total Area}_{\text{doors}} = 2 \times 3 = 6 \text{ m}^2 \]

### Step 4: Calculate the area occupied by the cement and sand mixture
- It is given that one-ninth of the wall area is occupied by the mixture:
\[ \text{Area}_{\text{mixture}} = \frac{1}{9} \times 321 = 36 \text{ m}^2 \]

### Step 5: Calculate the effective area of the wall
- Subtract the area of the doors and the area occupied by the mixture from the total wall area:
\[ \text{Effective Area} = \text{Total Area}_{\text{wall}} - \text{Total Area}_{\text{doors}} - \text{Area}_{\text{mixture}} \]
\[ = 321 - 6 - 36 = 279 \text{ m}^2 \]

### Step 6: Calculate the area of one brick
- The area of one brick can be calculated as:
\[ \text{Area}_{\text{brick}} = \text{Length} \times \text{Height} = 0.2 \times 0.08 = 0.016 \text{ m}^2 \]

### Step 7: Calculate the number of bricks required
- To find the number of bricks needed, divide the effective wall area by the area of one brick:
\[ \text{Number of bricks} = \frac{\text{Effective Area}}{\text{Area}_{\text{brick}}} = \frac{279}{0.016} \approx 3994 \]
- Rounding this gives approximately 400 bricks.

### Final Answer
Approximately **400 bricks** will be required to build the wall.