The volume of a wall which is 5 times as high as it is broad and 8 times as long as it is high, is 12.8 m³. The breadth of the wall is

40 cm

The wall is 5 times as high as it is broad, so h = 5b

The wall is 8 times as long as it is high, so l = 8h

 volume of the wall is 12.8 m³  = 12800000 cm³  ( given )

Volume = Length × Height × Breadth

12800000 = l × h × b

Now, substitute the expressions for "l" and "h" from the relationships above:

12800000 = (8h) × 5b × b

12800000 = (8 × 5b) × 5b × b

12800000 = 200 × b³

64000 = b³

b = 40 cm