1669. What character can be added to both of these characters to give other characters? 火_ -> _頁

Answer:

火  KA, hi - fire

頁  KETSU, peeji - page

Add 丁:火+丁 丁+頁 

丁  CHOU, TEI - counter, block

灯  TOU, hi - lamp, light

頂  CHOU, itadaki, itada(ku) - top, peak, summit, receive