Explanation:

- 1 horse can graze the field in 140 days (10 horses take 14 days).

- 1 sheep can graze the field in 280 days (10 sheep take 28 days).

- 5 horses can graze the field in 28 days (since they are half the number of horses).

- Horse's daily work: 1/140. Thus, 5 horses: 5/140 = 1/28.

- 10 sheep can graze the field in 28 days (since they are all sheep).

- Sheep's daily work: 1/280. Thus, 10 sheep: 10/280 = 1/28.

- Together, 5 horses and 10 sheep: 1/28 + 1/28 = 1/14.

- So, 5 horses and 10 sheep will take 14 days to graze the field.

- Option 3: 14 days is correct.