The bankers discount of a certain sum of money is Rs. 18 and the true discount on the same sum for the same time is Rs. 15. The sum due is:  Choose the correct choice from the following:
A) 89	B) 90	C) 91	D) 92	E) 93

Rationale: "sum = ( b . d * t . d ) / ( b . d - t . d ) ( 18 * 15 ) / 18 - 15 ; 90 answer : b"
Answer: B