Let S be the set of integers x such that:
1) 100 <= x <= 200
2) x is odd
3) x is divisible by 3 but not by 7.
How many elements does S contain?
Number of multiples of 3 between 100 and 200 = 66 - 33 = 33
Number of odd multiples = 16
Number of odd multiples of 21 = 3 (105, 147, 189)
So, the required number = 13
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