I want to calculate h.b.a interest but don't know how?

My father Expired in Service on 02/01/2013, He was Central Government Employee G.P 5400, He took H.B.A Advance Rs 295000 in year 09/1998, on interest @ 11% p.a, His Principle amount is fully repaid or say recovered in 06/2011, I want to know how much Interest amount is accumulated. He has to complete the H.B.A interest in 50 installments and he was paying Rs 2000 from 07/2011, he has already paid 18 installments(Rs 36000) till now, I just can not get how the interest rate of 11% p.a will give me the total interest amount. Please some one help me.