Let $r$ be the set of primitive $42^{\text{nd}}$ roots of unity, and let $s$ be the set of primitive $70^{\text{th}}$ roots of unity. how many elements do $r$ and $s$ have in common?

Liz is shipping a package to her friend.the maximum weight of a medium flat-rate box is 20 pounds,including the box which weighs 6 ounces.write and solve an inequality to describe the possible weight ,w, of the contents that liz will ship to her friend if she must use a medium flat-rate box.