REM Choose four random digits (1-9) with repetitions allowed:
      DIM digits%(4), check%(4)
      FOR choice% = 1 TO 4
        digits%(choice%) = RND(9)
      NEXT choice%

      REM Prompt the player:
      PRINT "Enter an equation (using all of, and only, the single digits ";
      FOR index% = 1 TO 4
        PRINT ; digits%(index%) ;
        IF index%<>4 PRINT " " ;
      NEXT
      PRINT ")"
      PRINT "which evaluates to exactly 24.  Only multiplication (*), division (/),"
      PRINT "addition (+) & subtraction (-) operations and parentheses are allowed:"
      INPUT "24 = " equation$

      REPEAT

        REM Check that the correct digits are used:
        check%() = 0
        FOR char% = 1 TO LEN(equation$)
          digit% = INSTR("0123456789", MID$(equation$, char%, 1)) - 1
          IF digit% >= 0 THEN
            FOR index% = 1 TO 4
              IF digit% = digits%(index%) THEN
                IF NOT check%(index%) check%(index%) = TRUE : EXIT FOR
              ENDIF
            NEXT index%
            IF index% > 4 THEN
              PRINT "Sorry, you used the illegal digit "; digit%
              EXIT REPEAT
            ENDIF
          ENDIF
        NEXT char%

        FOR index% = 1 TO 4
          IF NOT check%(index%) THEN
            PRINT "Sorry, you failed to use the digit " ; digits%(index%)
            EXIT REPEAT
          ENDIF
        NEXT index%

        REM Check that no pairs of digits are used:
        FOR pair% = 11 TO 99
          IF INSTR(equation$, STR$(pair%)) THEN
            PRINT "Sorry, you may not use a pair of digits "; pair%
            EXIT REPEAT
          ENDIF
        NEXT pair%

        REM Check whether the equation evaluates to 24:
        ON ERROR LOCAL PRINT "Sorry, there was an error in the equation" : EXIT REPEAT
        result = EVAL(equation$)
        RESTORE ERROR
        IF result = 24 THEN
          PRINT "Congratulations, you succeeded in the task!"
        ELSE
          PRINT "Sorry, your equation evaluated to " ; result " rather than 24!"
        ENDIF

      UNTIL TRUE

      INPUT '"Play again", answer$
      IF LEFT$(answer$,1) = "y" OR LEFT$(answer$,1) = "Y" THEN CLS : RUN
      QUIT