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# 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 $n,\ n\ +\ 4,\ n\ +\ 8,\ n\ +\ 12$ and $n\ +\ 16$. $n$ is any positive integer.

**To do:**Here we have to show that one and only one out of $n$, $n+4$, $n+8$, $n+12$ and $n+16$ is divisible by 5.

**Solution:**

Let $n\ =\ 5q\ +\ r$, where $0\ \underline{< }\ r\ <\ 5$.

So, $r\ =\ 0,\ 1,\ 2,\ 3,\ 4$.

If $r\ =\ 0$ then, $n\ =\ 5q$. So,

$n\ =\ 5q$,

**which is divisible by 5**.$n\ +\ 4\ =\ 5q\ +\ 4$, which is not divisible by 5.

$n\ +\ 8\ =\ 5q\ +\ 8$, which is not divisible by 5.

$n\ +\ 12\ =\ 5q\ +\ 12$, which is not divisible by 5.

$n\ +\ 16\ =\ 5q\ +\ 16$, which is not divisible by 5.

If $r\ =\ 1$ then, $n\ =\ 5q\ +\ 1$. So,

$n\ =\ 5q\ +\ 1$, which is not divisible by 5.

$n\ +\ 4\ =\ 5q\ +\ 1\ +\ 4\ =\ 5q\ +\ 5$,

**which is divisible by 5**.$n\ +\ 8\ =\ 5q\ +\ 1\ +\ 8\ =\ 5q\ +\ 9$, which is not divisible by 5.

$n\ +\ 12\ =\ 5q\ +\ 1\ +\ 12\ =\ 5q\ +\ 13$, which is not divisible by 5.

$n\ +\ 16\ =\ 5q\ +\ 1\ +\ 16\ =\ 5q\ +\ 17$, which is not divisible by 5.

If $r\ =\ 2$ then, $n\ =\ 5q\ +\ 2$. So,

$n\ =\ 5q\ +\ 2$, which is not divisible by 5.

$n\ +\ 4\ =\ 5q\ +\ 2\ +\ 4\ =\ 5q\ +\ 6$, which is not divisible by 5.

$n\ +\ 8\ =\ 5q\ +\ 2\ +\ 8\ =\ 5q\ +\ 10$,

**which is divisible by 5**.$n\ +\ 12\ =\ 5q\ +\ 2\ +\ 12\ =\ 5q\ +\ 14$, which is not divisible by 5.

$n\ +\ 16\ =\ 5q\ +\ 2\ +\ 16\ =\ 5q\ +\ 18$, which is not divisible by 5.

If $r\ =\ 3$ then, $n\ =\ 5q\ +\ 3$. So,

$n\ =\ 5q\ +\ 3$, which is not divisible by 5.

$n\ +\ 4\ =\ 5q\ +\ 3\ +\ 4\ =\ 5q\ +\ 7$, which is not divisible by 5.

$n\ +\ 8\ =\ 5q\ +\ 3\ +\ 8\ =\ 5q\ +\ 11$, which is not divisible by 5.

$n\ +\ 12\ =\ 5q\ +\ 3\ +\ 12\ =\ 5q\ +\ 15$,

**which is divisible by 5**.$n\ +\ 16\ =\ 5q\ +\ 3\ +\ 16\ =\ 5q\ +\ 19$, which is not divisible by 5.

If $r\ =\ 4$ then, $n\ =\ 5q\ +\ 4$. So,

$n\ =\ 5q\ +\ 4$, which is not divisible by 5.

$n\ +\ 4\ =\ 5q\ +\ 4\ +\ 4\ =\ 5q\ +\ 8$, which is not divisible by 5.

$n\ +\ 8\ =\ 5q\ +\ 4\ +\ 8\ =\ 5q\ +\ 12$, which is not divisible by 5.

$n\ +\ 12\ =\ 5q\ +\ 4\ +\ 12\ =\ 5q\ +\ 16$, which is not divisible by 5.

$n\ +\ 16\ =\ 5q\ +\ 4\ +\ 16\ =\ 5q\ +\ 20$,

**which is divisible by 5**.Hence, in each case, only one out of $n,\ n\ +\ 4,\ n\ +\ 8,\ n\ +\ 12$ and $n\ +\ 16$ is divisible by 5.

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