Show that one and only one out of n, n + 4, n + 8, n + 12 and n + 16 is divisible by 5 where n is any positive integer.


Given numbers are n, (n + 4), (n + 8), (n + 12) and (n + 16) where n is any positive integer.


Then let n = 5q, 5q + 1, 5q + 2, 5q + 3, 5q + 4 for q є N


Hence, one and only one out of n, n + 4, n + 8, n + 12 and n + 16 is divisible by 5.


Then n = 5q + r, where 0 ≤ r < 5.


n = 5q or 5q + 1 or 5q + 2 or 5q + 3 or 5q + 4.


Case I If n = 5q, then n is only divisible by 5.


So, in this case, n is only divisible by 5.


Case II If n = 5q + 1, then n + 4 = 5q + 1 + 4 = 5q + 5 = 5(q + 1) which is only divisible by 5.


So, in this case, (n + 4) is only divisible by 5.


Case III If n = 5q + 2, then n + 8 = 5q + 2 + 8 = 5q + 10 = 5(q + 2) which is only divisible by 5.


So, in this case, (n + 8) is only divisible by 5.


Case IV If n = 5q + 3, then n + 12 = 5q + 3 + 12 = 5q + 15 = 5(q + 3) which is only divisible by 5


So, in this case, (n + 12) is only divisible by 5.


Case V If n = 5q + 4, then n + 16 = 5q + 20 = 5(q + 4), which is divisible by 5.


So, in this case, (n + 16) is only divisible by 5.


Hence, one and only one of n, n + 4, n + 8 n + 8, n + 12 and n + 16 is divisible by 5 where any positive integer is an integer.


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