The noble gases are the least of any elements on the periodic table. form acidic compounds with hydrogen. the halogens are the most reactive among all .

Explanation :

Noble gases are the elements of group 18. This series contains helium, neon, radon krypton and radon.

The noble gases are the least reactive of any elements on the periodic table. This is because they have eight valance electrons in its outermost shell.

Non-metals or poly atomic ions forms  acidic compounds with hydrogen. For example : etc forms acidic compounds with hydrogen.

The halogens are the most reactive among all the elements. This is because halogens suddenly combines with most of the metals. They are never seen uncombined in nature.

For example : Chlorine, fluorine and astatine are some halogens.

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