The key to answering "word problems" of this sort is being able to translate the given information into mathematical operations involving the data provided. Key words here are "of" and "about".
"About one-sixth of the (25) invitations sent were for her family."
⟺About16×⏟OF25 ⟺ About 1 6 × ⏟ OF 25 of the invitations were for her family.
⟺16×25=4.166⎯⎯⎯⎯⎯⎯≈⏟about4 ⟺ 1 6 × 25 = 4.1 66 ¯ ≈ ⏟ about 4 invitations were for her family.
