Volume of water in Tank Y at first
= 4/5 * capacity of container
= 4/5 * (length x width * height)
= 4/5 * (20 * 30 * 100)
= 48 000 cm^3

For every 2 cm height of water in Tank X, volume of water
= length * width * height
= 30 * 30 * 2
= 1 800 cm^3

For every 1 cm height of water in Tank Y, volume of water
= length * width * height
= 20 * 30 * 1
= 600 cm^3

This totals 1 800 + 600 = 2 400 cm^3 of water.

Since there is 48 000 cm^3 of water at first. number of complete sets of 2 400 cm^3 that we can achieve
= 48 000 / 2 400
= 20

This means that there are 20 sets of 2 cm heights in Tank X and 20 sets of 1 cm heights in Tank Y.

Height of water in Tank X after the transfer
= 20 * 2
= 40 cm