A sum of money invested at simple interest amounts to Rs.720 at the end of two years and to Rs.1020 at the end of another 5 years. What is the principal invested and at what rate of interest was it invested?

Explanation:

For a period of 5 years, simple interest amounts to Rs.1020 – Rs.720 = Rs.300

Simple Interest per year = 300\5 = Rs.60

Therefore, Simple Interest for 2 years = Rs.120

=> Principal amount = 720 – 120 = Rs.600

SI=Pnr\100=>600×2×r\2=120orr=10