Question regarding capital gain on sale of house property

Mr X purchased a flat in 1991 and sold it in 2019. Long term capital gain arised was invested in capital gain scheme of public sector bank in March 2021 as sale proceeds were received in October 2020.

Now Mr X is intended to purchase same flat using capital gain. Is that possible?

Also if Mr X is intended to sell same flat in 6 months how capital gain will be calculated?