Volume of brick = 24 cm × 12 cm × 8 cm
Volume of wall    = 24 m × 8 m × 6 m
            = (24 × 8 × 6) × 106  cm³
Volume left which will be filled by bricks 
 $=\frac{80}{100}\times (24\times 8\times 6)\times {10}^6{\mathrm{cm}}^3$
 Number of bricks $=\frac{8\times {10}^5\times 24\times 8\times 6}{24\times 12\times 8}$
                    = 400000