Using a periodic table, explain why elements in group 17 are more reactive than those in group 18.

Group 18 elements are noble gasses which are stable. They have a full valence shell so they are don't typically react too much. Group 17 only needs one more valence electron so these are most often paired with group 1 elements. Group 17 are Halogens and most often react with Alkali metals. Example: NaCl (Na group 1 | Cl group 17)

Explanation:

d.7.5cm is your answer

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