You can simply multiply all the combinations of 3,5, and 7 in pairs:
1) 3 * 5 = 15
2) 3 * 7 = 21
3) 5 * 7 = 35
Don't forget the numbers multiplied to itself:
4) 3 * 3 = 9
5) 5 * 5 = 25
6) 7 * 7 = 49
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