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Answer: 105

     21  and  35 are factors
so 3, 7 and 5, 7 are prime factors

If 3, 5 and 7 are the only prime factors then the factors are:
1, 3, 5, 7,
15, 21, 35, and the number itself

The number itself = 3$\times$5$\times$7 = 105



This problem is taken from the UKMT Mathematical Challenges.
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