The simple interest and the true discount on a certain sum for a given time and at a given rate are Rs. 90 and Rs. 80 respectively. The sum is:	Answers.
A) 1360  B) 1450  C) 720  D) 1800  E) none

Rationale: "sol . sum = s . i . * t . d . / ( s . i ) - ( t . d . ) = 90 * 80 / ( 90 - 80 ) = rs . 720 . answer c"
Answer: C