Amount of milk in 80% concentrated milk mixture of 640 litres is = 0.8 *640=512 litres.

Let X litres of milk be added to the 80% milk mixture to to get concentration between 65% and 70%.

Amount of milk in 65% milk mixture = (65/100 )* (640+X) litres.

Amount of milk in 70% milk mixture = (70/100 )* (640+X) litres

(65/100)* (640+X) < 512 < 70/100(640+X)

=> 0.65(640+X) < 512 < 0.70(640+X)

=> 416 + 0.65 X < 512 < 448 + 0.70 X

=> 512 > 416 + 0.65 X and 448+ 0.70X > 512

=> 96 > 0.65 X and 0.7 X > 64

=> X < 96/0.65 and X > 64/0.7

=> X < 147.69 and X > 91.43