I. Of the two sisters, Mary is the YOUNGEST and Anna, the OLDEST.

Of the two sisters, Mary is the YOUNGER and Anna, the ELDER [OLDER].

II. I want to work for someone THAT challenges me.

I want to work for someone WHO challenges me.

III. We’re hiring a new person so everyone has LESS cases.

We’re hiring a new person so everyone has FEWER cases.

IV. She finished it in LIKE, LITERALLY, three hours.

She finished it in three hours.