Given the following sum: $•+1•+2•+•3+•1=21•$ We need to determine what the symbol "•" represents.

Solution: Let "•" be denoted by $n$. Rewriting the equation in terms of $n$: $n+1n+2n+n3+n1=21n$ Now, expand each term: $n+10+n+20+n+10n+3+10n+1=210+n$ Simplifying the equation: $23n+34=210+n$ Subtract $n$ from both sides: $22n+34=210$ Now, subtract 34 from both sides:

$22n=176$ Solve for $n$: $n=8$.

Conclusion: The symbol "•" represents the digit 8.