Explanation:

- A can complete the work in 15 days, so A’s one day work is 1/15.

- B can complete the work in 25 days, so B’s one day work is 1/25.

- Together A and B’s one day work is $\frac{1}{15}+\frac{1}{25}=\frac{8}{75}$.

- In 5 days, A and B can do $\frac{8}{75}\times 5=\frac{40}{75}=\frac{8}{15}$ of the work.

- Remaining work is $1-\frac{8}{15}=\frac{7}{15}$.

- A and C complete this remaining work in 4 days, so their one day work is $\frac{7}{15}÷4=\frac{7}{60}$.

- A's one day work is 1/15, so C's one day work is $\frac{7}{60}-\frac{1}{15}=\frac{1}{20}$.

- Therefore, C alone can complete the work in 20 days.

Option 2: 20 days is correct.

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