Because each person's food level does not change, we can assume that each person's rice level everyday is a.

Therefore, at the beginning, the army unit has 200 x 30 x a = 6000a rice level.

After 10 days, the unit remains 6000a - 200 x 10 x a =4000a rice level 

and has 200 + 50 = 250 people.

Thus, the number of days that the unit has rice to eat is 4000a /(250 x a) = 16.

So, the rice is left for the unit to eat in 16 days. 

Because each person's food level does not change, we can assume that each person's rice level everyday is a.

Therefore, at the beginning, the army unit has 200 x 30 x a = 6000a rice level.

After 10 days, the unit remains 6000a - 200 x 10 x a =4000a rice level 

and has 200 + 50 = 250 people.

Thus, the number of days that the unit has rice to eat is 4000a /(250 x a) = 16.

So, the rice is left for the unit to eat in 16 days.