Exemption u/s 54

Suppose on sell of a House Property, the long term capital gain comes up to Rs. 20 lac for the assessee. Now, the assessee purchases a new asset worth Rs. 30 lac as a joint property with 50% share and takes a loan of Rs. 15 lac for the same. 

The query which arises in respect of the above is what amount shall be eligible as exemption u/s 54 for the assessee.