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Answer:
4
Step-by-step explanation:
this is the same question as what number can divide
13-1 = 12, 17-1 = 16 and 21-1 = 20 and has 0 remainder ?
the greatest number that can do that is 4.
we can easily see that, but formally, let's do prime factorization :
12 ÷ 2 = 6
6 ÷ 2 = 3
3 ÷ 2 no
3 ÷ 3 = 1 finished
12 = 2×2×3
16 ÷ 2 = 8
8 ÷ 2 = 4
4 ÷ 2 = 2
2 ÷ 2 = 1 finished
16 = 2×2×2×2
20 ÷ 2 = 10
10 ÷ 2 = 5
5 ÷ 2 no
5 ÷ 3 no
5 ÷ 5 = 1 finished
20 = 2×2×5
so, the largest common factor is the combination of the longest streaks per factor they have in common.
they only have 2s in common.
and the longest common streak is 2×2 = 4.
hence the answer