Long term capital gain

Yes, you are liable to capital gain tax @ 20% plus surcharge (if applicable) & cess.

Since you have sold the said flat in the current Year 2015-16, there will be a long term capital gain of Rs.5,99,167/-,worked out as follows, as the period of 36 months from the date of purchase to the date of sale of the said flat already exceeded:

Sales Consideration                                   = Rs. 15,00,000  (assumed to be equal to or more than stamp duty value)

Less : Indexed cost of acquisition

         ( 4 lakhs × 1081 / 480)                         = Rs.  9,00,833

LTCG                                                          = Rs. 5,99,167

Note:

(1) Purchase of flat assumed to be within April, 2004 to March, 2005 as CII of F.Y. 2004-05.has been considered.

(2) However you can claim exemption of capital gain u/s-54 & 54EC of the Income Tax Act, 1961, if you fulfil the required conditions.

(3) You can also claim exemption of LTCG  u/s-112(1)(a), if your total taxable income minus LTCG is less than the exemption limit applicable in your case to the extent as below:   Exemption Limit - (Taxable Income - LTCG)

Yes you to pay LTCG at 20%

Yes LTCG is required to be paid @ 20%.However, if you are not having any income other than the LTCG, you can get an addition exemption of 2,50,000 from the LTCG earned. In this way assuming that you are not having other incomes, your LTCG tax liability comes to (5,99,167***-2,50,000)*.20 = 69,833. Eventhough you are having other incomes but less than 2,50,000 you can claim the difference amount as a deduction from the LTCG.

***As Mr.stranger worked out the calculation of LTCG, I did not specificaly mentioned the calculation of LTCG