Dear Sir,
You have to take the fair market value of the property as on 1 April 2001. That fair market value will be considered as your purchase price and accordingly you have to calculate capital gain by deducting fair market value from the sale price.
Your capital gain would be 1/4 of the total capital gain.
Please feel free to call/ revert in case of any doubts
Thanks and Regards
Abhishek Dugar
CA CS B.Com