Selling of a long term asset

respected sir, 
if mr. X sells his property :
amount received after selling the property 30 lakhs 
transfer fees and legal expenditure : 1 lakh
cost of acquisition: 15 lakhs
cost of improvement : 2 lakhs 
gross sale consideration is rupees 12 lakh so now if mr X wants too invest in bonds or put in the capital gain account scheme then should he invest the whole 12 lakhs or nly the tax @ 20 % i.e.. 2 lakh 40 thousand ??