The answer is $72.90.
Change 9% into a decimal by dividing by 100. You get 0.09.
Multiply 0.09 by 810. You get 72.90 which is the answer.
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[tex] \bf a\%~of~b \implies \cfrac{ a}{100}\cdot b~\hfill \textit{9\% of 810 is }\cfrac{9}{100}\cdot 810\\ ~\hspace{34em} [/tex]
