Here you go :)
SCORE=0
string to store the score in
GAMESCORE$=""
puts the score into the string
GAMESCORE$=STR$(SCORE)
saves it into a file
SAVE"TXT:SCORE",GAMESCORE$
and to load it back
LOAD"TXT:SCORE" OUT GAMESCORE$
SCORE=VAL(GAMESCORE$)
4そうだね プレイ済み