Here's the solution,
Volume of wall :
=》total bricks × volume of each brick
=》1200 × 0.075 m³
=》90 m³
volume of wall :
=》lenght × width × height
=》25 × 9 × width
=》225 × width
now, by equating the volumes, we get
( since they are volume of same wall )
=》225 × width = 90
=》width = 90 ÷ 225
=》width of wall = 0.4 m