Why will the volume remain same in both Qty A and B i.e. 60000?

I don’t know how you arrived at 60,000, but I think the answer might be B, i.e The quantity in column B is greater

It’s not asking about the volume of water, it’s asking about the height of water

Let’s say the total volume of water = 6000

Then height in A = \frac{6000}{30 * 50} = 4

Whereas height in B = \frac{6000}{30 * 40} = 5

Thus, quantity B is greater