If Rs. 64 amount to Rs. 83.20 in 2 years, what will Rs. 86 amount to in 4 years at the same rate percent per annum ?  Choose the correct choice from the following answers:
A) Rs. 114.80
B) Rs. 86.70
C) Rs. 55.40
D) Rs. 51.60
E) None
Solution   P = Rs. 64,S.I. = Rs. (83.20 - 64) = Rs. 19.20, T= 2 years.   So, rate = (100x19.20/64x2)= 15%.   Now, P = Rs. 86, R = 15% ,T = 4 years.   ∴ S.I. = Rs.(86x15x4/100) Rs. 51.60.   Answer Rs. 51.60
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If Rs. 64 amount to Rs. 83.20 in 2 years, what will Rs. 86 amount to in 4 years at the same rate percent per annum ?