Plz solve 13th question

Solution) 13) 
Length of the wall = 5 m = 500 cm   {1 m = 100 cm}
Breadth of the wall = 20 cm
Height of the wall = 2 m = 200 cm
Volume of wall =  500 × 20 × 200 = 2000000 cm^{​​​​​3}
Now, volume of each brick = 20 × 10 × 4.5 = 900 cm³
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Also, volume of the mortar = $\frac{1}{10}\times$ (Volume of wall)
                                                = $\frac{1}{10}$ × 2000000
                                                =  200000 cm^{​​​​​3​​​​​​}
Total volume of the bricks in the wall 
= volume of the wall − volume of motor
= (2000000 − 200000) cm³
= 1800000 cm³

∴ Number of bricks = $\frac{volume of bricks}{volume of each brick}=\frac{1800000}{900}=2000 bricks$