Find the total volume of water first

90cm² x 35cm = 3150cm³

Let new height of water in Y be 1 unit.
So new height of water in X is 4 units.

Total volume of water
= 1 unit x 60cm² + 4 units x 90cm²
60cm² units + 360cm² units
= 420cm² units

420cm² units = 3150cm³,

so 1 unit = 3150cm³ ÷ 420cm²
= 7.5cm

New height of water level in X
= 7.5cm x 4
= 30cm

Alternatively, ratio of base area of X to Y
= 90 : 60
= 9 : 6
= 3 : 2

Since X will have 4 times the height of Y,

3 x 4 = 12
2 x 1 = 2

Ratio the volume of water in X : in Y

= 12 : 2
= 6 : 1

Total = 7 units

Volume of water = 3150cm³ = 7 units
1 unit = 3150 ÷ 7
= 450cm³

Height in X
= 6 x 450cm³ ÷ 90cm²
= 6 x 5cm
= 30cm