Your total C.G is Rs. 32L from which you purchased H.P of 15L + 11L = 26L remaining 6L according two you, but you can claim the expemtion against one house only as u have sold only 1 house. However, two or more residential houses purchased can be classified as one single residential house, the exemption under section 54 can be allowed. Some of the relevant judicial pronouncements are
1)**[D. Anand Basappa v. ITO 309 ITR 329 (Kar.) (2009)]**
2)**Two adjoining flats converted into single residence, exemption allowed. [ACIT v Mrs. Leela P. Nanda 286 ITR (AT) 113 (Mum) (2006)]**
But as u said that you are taking the exemption of Rs. 15L u/s 54 so remaining capital gain is Rs. 17L there are two ways to
1) Invest in bonds, eligible bonds RECL (Rural Electrification Corporation Ltd), NHAI (National Highways Authority of India) invest can be made upto 50L, but the invetment to be made within 6 months of sale of property.
2) Sec 54 GB :- You can subscribe in the equity shares of an eligible company before the due date of furnishing of return of income under sub-section (1) of section 139.