
Answer:
 24 cm
Step-by-step explanation:
The product of height and width will remain the same, so the new width (w) will be given by ...
 w·(new height) = (old width)·(old height)
 w·(1.25·40 cm) = (30 cm)(40 cm)
 w = (30 cm)/1.25 = 24 cm . . . . . . . divide by 1.25·40 cm and simplify
_____
This derives from the fact that volume and length are unchanged. The formula for the volume in terms of length, width, and height is ...
 V = LWH
Then the product of W and H is the constant ...
 V/L = WH . . . . . divide by L
For our purpose, we only need to know that V and L are unchanged, so the product WH is unchanged. We don't need to know their values.
Of course, increasing the height by 25% is equivalent to multiplying it by 1.25:
 H + 25/100·H = H·(1 + 0.25) = 1.25H