Answer
$1.8\times10^{-3}M/s$
Work Step by Step
$r_{av}= -\frac{1}{2}\times\frac{\Delta [HBr]}{\Delta t}$
$=-\frac{1}{2}\times\frac{0.512M-0.600M}{(25-0)s}= 1.8\times10^{-3}M/s$
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