It is 8499.... It is the biggest 4- round down so 8400 And there's no more number bigger than 8499 to round to 8400

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There's\ no\ need\ for\ the\ solution. \\ In\ this\ situation,all\ you\ have\ to\ do\ is\ \underline{trial\ and\ error} \\ \\ \underline{8,499\ is\ the\ answer}. \\ \\ Checking; \\ \\ \boxed{8,\underline{\underline{4}}99}\ therefore,\underline{round\ down} \\ \\ 8,499\to\boxed{\boxed{8,000}}. \\ \\ Hope\ it\ Helps:) \\ Domini