The S.I. on a certain sum in 3 years and at 8% per year is ₹ 720. The sum is:

1. ₹ 6,000

2. ₹ 9,000

3. ₹ 3,000

4. ₹ 4,000

Given:

T = 3 years

R = 8%

S.I. = ₹ 720

Let the principal be $P$.

As we know,

$\text{S.I.} = \Big(\dfrac{P \times R \times T}{100}\Big)\\[1em] \Rightarrow 720 = \Big(\dfrac{P \times 8 \times 3}{100}\Big)\\[1em] \Rightarrow 720 = \Big(\dfrac{24P}{100}\Big)\\[1em] \Rightarrow 720 = \Big(\dfrac{6P}{25}\Big)\\[1em] \Rightarrow P = \Big(\dfrac{25 \times 720}{6}\Big)\\[1em] \Rightarrow P = \Big(\dfrac{18,000}{6}\Big)\\[1em] \Rightarrow P = 3,000$

Hence, option 3 is the correct option.