A wall is to be constructed with bricks of dimensions 25 cm x 10 cm x 8 cm. The wall's dimensions are 9.9 m x 0.2 m x 0.3 m. If 10% of the wall's volume is cement, how many bricks are needed?

Given dimensions of one brick:
Length ($l_b$) = $25 \text{ cm}$
Width ($w_b$) = $10 \text{ cm}$
Height ($h_b$) = $8 \text{ cm}$

Given dimensions of the wall:
Length ($L_w$) = $9.9 \text{ m} = 9.9 \times 100 \text{ cm} = 990 \text{ cm}$
Thickness ($W_w$) = $20 \text{ cm}$
Height ($H_w$) = $30 \text{ cm}$

The volume of a brick ($V_b$) is given by the product of its length, width, and height.

$$V_b = l_b \times w_b \times h_b$$ $$V_b = 25 \text{ cm} \times 10 \text{ cm} \times 8 \text{ cm}$$ $$V_b = 2000 \text{ cm}^3$$

The volume of the wall ($V_w$) is given by the product of its length, thickness, and height.

$$V_w = L_w \times W_w \times H_w$$ $$V_w = 990 \text{ cm} \times 20 \text{ cm} \times 30 \text{ cm}$$ $$V_w = 594000 \text{ cm}^3$$

The problem states that $10\%$ of the wall's volume is filled with cement (mortar). This means the remaining $90\%$ of the wall's volume is occupied by bricks.
Volume occupied by bricks ($V_{bricks\_in\_wall}$) = $90\%$ of $V_w$

$$V_{bricks\_in\_wall} = 0.90 \times V_w$$ $$V_{bricks\_in\_wall} = 0.90 \times 594000 \text{ cm}^3$$ $$V_{bricks\_in\_wall} = 534600 \text{ cm}^3$$

The number of bricks ($N$) is the total volume occupied by bricks in the wall divided by the volume of one brick.

$$N = \frac{V_{bricks\_in\_wall}}{V_b}$$ $$N = \frac{534600 \text{ cm}^3}{2000 \text{ cm}^3}$$ $$N = 267.3$$

The number of bricks required is $\text{267.3}$.