Answer:
FOR i% = 1 TO 100
IF ((i%\3) = i%/3) AND ((i%\7) = i%/7) THEN
PRINT i%
END IF
NEXT i%
Explanation:
Of course using MOD would be cleaner, but another way to check if a number is integer divisable is to compare the outcome of an integer division to the outcome of a floating-point division. If they are equal, the division is an integer division.
The program outputs:
21
42
63
84
