Question

Naveen lent Rs. 25000 to his two friends. He gave Rs. 10,000 at 10% per annum to one
of his friend and the remaining to other at 12% per annum. How much interest did he
receive after 2 years.

Answer 1

Naveen has 25,000 rupees,in which he gave 10,000 rupees to  friend (A) at 10% per annum for two year,then
S.I of 10,000 rupees in two years = (10000×2×10) ÷ 100 = 2000 rupees
Naveen gave 15,000 rupees to other friend for two years at 12% per annum,then
S.I of 15000 rupees in two years = (15000×2×12) ÷ 100 = 3600 rupees
therefore total S.I  = 2000+3600 = 5600 rupees
This is your answer on S.I

If we will have to find compound interest in two year of this amount,then we have to find S.I of two years for both friends on their rates.
Because their principal amount (P)for last one year will become = (main amount) + (S.I of first year)
so,,  S.I of last year for friend (A) = (P×T×R) ÷ 100
        S.I   =   (main amount ,which given to friend(A) +S.I of A in first year )×(1 year)×(the rate at which amount was given to A) ÷ (100)
S.I of last year for friend A = {( 10000+(10000×1×10 ÷ 100)}.×(1)×(10) ÷100
             = (10000+1000)×(1)×(10) ÷ 100
             = 11000×1×10 ÷ 100 
S.I of last year for friend A = 1100
therefore C .I  of two year for friend A = sum of S.I of both years = 1000+ 1100 = 2100 rupees
Now similar for other friend(B) bt this time rate will be 12% per annum and amount is 15000 rupees
S.I of 1st year = 15000×1×12÷100 = 1800 rupees
now principal amount for last year will become 15000 + 1800 = 16800 rupees
S.I of last year for B = 16800×1×12÷100 = 2016 rupees
Therefore C.I of two year for friend B = sum of S.I of both annum 
                                                           =1800+2016 = 3816 rupees
 Finally total compound interest (C.I) = C.I of two year for one friend(A) + C.I of two years for other friend (B)
total C.I = 2100 + 3816 = 5916  rupees
This is your answer on C.I