A lent Rs. 5000 to B for 2 years and Rs. 3000 to C for 4 years on simple interest at the same rate of interest and received Rs. 2640 in all from both of them as interest. The rate of interest per annum is? Choose the correct answer:  A) 16 %  B) 12 %  C) 74 %  D) 10 %  E) 45 %

Rationale: "let the rate be r % p . a . then , ( 5000 * r * 2 ) / 100 + ( 3000 * r * 4 ) / 100 = 2640 100 r + 120 r = 2640 r = 12 % answer : b"
Answer: B