A sum of money lent at simple interest amounts to Rs. 720 after 2 years and to Rs. 1,020 after a period of 5 years. What is the Principal?

Amount after 2 years = Rs. 720

Amount after 5 years  = Rs. 1,020

Interest is simple interest. 

Formula Used: 

​$$S.I = \frac{P \times r \times t}{100}$$ 

Where, P = principal, r = rate of interest, t = time.

Solution: 

Simple interest for 3  year = 1020 - 720 = 300. 

Now, Simple interest for one year  = $$\frac{300}{3} = 100$$ 

Simple interest for 2 years = $$2 \times 100 = 200$$​​

Now , Principal = Amount - Interest 

                          = 720 - 200

                          = 520 

Thus, the Principal is Rs. 520.