1580. What character can be added to both of these characters to give other characters? 其_ -> _宛

Answer:

其  shi, so - that

宛  ON, EN, ate(ru) - allocate

Add 月:其+月 月+宛 

月  GETSU, GATSU, tsuki - moon, month

期  KI, GO - time, period, term

腕  WAN, ude - arm, ability, talent, skill