We have to calculate the principal amount where S.I. is 10640, rate is 4%, and…

Method: use the simple interest formula and solve for the principal.

1. Formula for simple interest: SI = P × R × T / 100.

2. Rearrange to find principal: P = SI × 100 / (R × T).

3. Substitute the given values (SI = 10,640, R = 4%, T = 20 years): P = 10,640 × 100 / (4 × 20).

4. Compute the denominator: 4 × 20 = 80.

5. Compute the numerator: 10,640 × 100 = 1,064,000.

6. Divide: 1,064,000 ÷ 80 = 13,300.

7. Verify: Simple interest on 13,300 at 4% for 20 years = 13,300 × 4 × 20 / 100 = 10,640, which matches the given interest.

Answer: The principal amount is 13,300.