A sum of money grows to Rs. 3,640 in 6 years and Rs. 4,120 in 8 years under simple interest. What is the original principal amount?

Amount after 6 years = 3,640 Amount after 8 years = 4,120 Difference between the two amounts is only due to 2 extra years of interest: Extra interest for 2 years = 4,120 − 3,640 = 480 So, interest for 1 year = 480 ÷ 2 = 240 Under simple interest, interest per year is constant. Now use the 6-year amount: Total interest in 6 years = 6 × 240 = 1,440 Amount after 6 years = Principal + Interest ⇒ 3,640 = P + 1,440 ⇒ P = 3,640 − 1,440 = 2,200 Original principal = ₹2,200