Manish drove from Chicago to Baton Rouge. At 8:00 a.m., he was 510 km from Baton Rouge. At 1:00 p.m., he was 105 km from Baton Rouge. Which of the following values is closest to Manish's average speed, in kilometers per hour, from 8:00 a.m. to 1:00 p.m. ?

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5 hours passed between 8:00 a.m. and 1:00 p.m.

$$ d=rt $$ $$ r = \frac{d}{t} $$ $$ = \frac{510-105}{5} $$ $$ = \frac{405}{5} $$ $$ =\boxed{81} $$