( 1)

30kg food A, then 100-30=70kg food B. ..

The content of vitamin A in the mixed food: 30 * 400+70 * 600 = 54000 >; 44000, which meets the requirements;

Vitamin B content: 30 * 1000+70 * 200 = 44000.

Therefore, if we take 30kg of food A, we can't meet the specified requirements.

(2)

Let x kg be the amount of food A. According to the meaning of the question, the amount of food B is (100-x)kg.

According to the meaning of the question, list the inequality groups.

400x + 600( 100-x)>= 44000 ......

1000x + 200( 100-x)>= 48000 ......

Find the above set of inequalities and get 35.

Therefore, the value of food A in the range of [35,80] can meet the specified requirements.