capital gain

there is a case law in cap gain chapter-----C R SUBRAMANIAN (Kar.)                                                                                                

fact:- where land is purchased 3 yrs ago for rs.2 lakh and now a building is constructed on the land at cost of rs. 10 lakh. the said land and building is sold to a person for aggregate value of rs. 20 lakh

decision: it was held that for purpose of capital gain computation the land is long term capital asset and the building is short term cap asset.

my doubt: it is right that the land is long term capital asset and the building is short term cap asset. but in which ratio shall the sale consideration of rs 20 lakh be divided between long term land and short term building? 

thank for solving my query.

niranjan