Explanation:

- Here’s the thing: the sum grows from Rs 1020 in 5 years to Rs 1200 in 8 years—so, in those extra 3 years (from year 5 to year 8), the simple interest added is Rs 1200 - Rs 1020 = Rs 180.

- That means in 3 years, you’re earning Rs 180, so yearly simple interest = Rs 180 / 3 = Rs 60.

- In 5 years, total interest is 5 × 60 = Rs 300.

- So, the principal must be Rs 1020 (amount after 5 years) minus Rs 300 (interest for 5 years), which is Rs 720.

Option 4: 720 is the correct answer.

- Quick check on the other options:

- 820: Would mean interest of Rs 200 in 5 years, so interest per year Rs 40—not matching.

- 780 and 700: Their differences from Rs 1020 in 5 years don’t match the Rs 60/year calculation.