- Arshi completes the work in 30 days, so her 1 day's work is \( \frac{1}{30} \).
- Ahan completes it in 20 days, so his 1 day's work is \( \frac{1}{20} \).
- Together in 1 day, they do \( \frac{1}{30} + \frac{1}{20} = \frac{1}{12} \) of the work.
- In 4 days, together they complete \( 4 \times \frac{1}{12} = \frac{1}{3} \) of the work.
- Thus, the remaining work is \( 1 - \frac{1}{3} = \frac{2}{3} \).
- Arshi alone will complete the remaining \( \frac{2}{3} \) of the work.
- If Arshi does \( \frac{1}{30} \) per day, time taken is \( \frac{\frac{2}{3}}{\frac{1}{30}} = 20 \) days.
- **Correct Answer: Option 4 - 20 days** ✔️

YOUR ANSWER IS RIGHT