Description for Correct answer:
Let us assume that each soldier eats one unit of food per day. Thus, total units of food at the beginning will be

\( \Large 1000 \times 30 \) = 30000.

After 10 days 1000 soldiers would have eaten \( \Large 1000 \times 10 \) = 10000 units of food.

Thus, food left after 10 days equals 20000 units. Now, there are total of 2000 soldiers who eat one unit of food every day. So, the number of days that 20000 units of food will serve 2000 soldiers is \( \Large \frac{20000}{2000} \) = 10.