Cube B's sides are 25% longer. So if Cube A sides are taken as 100%, Cube B's will be 125% of Cube A's.
100% → 4cm
125% → 4cm x 125/100 = 4cm x 5/4 = 5cm
So Cube B has sides that are 5cm long
Or
25% longer means 1/4 longer. If Cube A's sides are 4 units long, then Cube B's sides are 1 unit longer
4 units = 4cm
1 unit = 1cm
Cube B 's sides = 1 unit + 4 units = 5 units
= 5 x 1cm = 5cm
Volume of Cube B = 5cm x 5cm x 5cm = 125cm³

b)
Volume of Cube A = 4cm x 4cm x 4cm = 64cm³
Total of cube A, B and C = 135cm³ x 3 = 405cm³
So volume of Cube C
= 405cm³ - 125cm³ - 64cm³ = 216cm³
216cm³ = 6cm x 6cm x 6cm
So Cube C has sides 6cm long.
6cm - 4cm = 2cm
So Cube C's sides are 2cm longer than Cube A's sides.
2cm/4cm x 100% = ½ x 100% = 50%
Cube C's sides are 50% longer than Cube A's sides.