a) Rs. 6651
b) Rs. 6615
c) Rs. 6516
d) Rs. 6156

Answer: b
Explanation: $$5\% = \frac{1}{{20}} = \frac{{21 \to {\text{ Installment}}}}{{20 \to {\text{ Principal}}}}$$

Year	Principal	Installment
⇒ I	20×21 →	21×21	……(i)
⇒ II	400→	441	…..(ii)

Since, installment is equal, hence multiply equation (i) by 21
Total principal = 420 + 400 = 820
820 units = Rs. 12300
1 units = Rs. 15
441 units = Rs. 6615
Each installment = Rs. 6615