If the amount obtained by A by investing Rs. 9,100 for three years at a rate of 10% p.a. on simple interest is equal to the amount obtained by B by investing a certain sum of money for five year at a rate of 8% p.a. on simple interest, then 90% of the sum invested by B (in Rs.) is:

Amount obtained by A is equal to amount obtained by B
Amount = Principal + Simple Interest
$$\eqalign{ & 9100 + \frac{{9100 \times 10 \times 3}}{{100}} = B + \frac{{B \times 8 \times 5}}{{100}} \cr & 9100 + 2730 = \frac{{140}}{{100}}B \cr & B = 11830 \times \frac{{100}}{{140}} \cr & B = 8450 \cr} $$
90% of invested amount by B
$$\eqalign{ & = 8450 \times \frac{{90}}{{100}} \cr & = 7605 \cr} $$