305 can only be [exactly] divided by 5 and not 2, 3, 6, 9 nor 10.
Divisibility tests:
Last digit is 5 which is not even, so the whole number (305) not divisible by 2.
3 + 0 + 5 = 8 which is not divisible by 3, so the original number (305) is not divisible by 3.
3 + 0 + 5 = 8 which is not divisible by 9, so the original number (305) is not divisible by 9.
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You get 10. 1/3 of 60=20 and 1/2 of 60=30, so 30-20=10.
Divide by two and three. 2 x 2 x 3 x 3 = 36
610 is divisible by these factors: 1, 2, 5, 10, 61, 122, 305, 610.
Divide by two until you get to three. (48,1)(24,2)(16,3)(12,4)(8,6)
72 Divide by two. 36,2 Divide by two. 18,2,2 Divide by two. 9,2,2,2 Divide by three. 3,3,2,2,2 Stop. 2 x 2 x 2 x 3 x 3 = 72