Given vector A with magnitude 2N and vector B magnitude 4N, what are the minimum and maximum magnitudes of A+B?​

-- If A and B are pointing in exactly opposite directions, then their sum A+B is the minimum possible value.  A+B = 2 N in the same direction as B.

-- If A and B are pointing in exactly the same direction, then their sum A+B is the maximum possible value.  A+B = 6 N in that same direction.  

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form