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:  Choose the correct choice from the following options:
A) 1360
B) 1450
C) 720
D) 1800
E) None
Sol.   Sum = S.I. * T.D. / (S.I)-(T.D.) = 90 * 80 / (90 - 80) = Rs.720.   Answer 720
<result>C</result>
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: