Verified Answer :

AL2006

-- Driving at 30 mph for 20 minutes (1/3 hour), you cover 10 miles.

-- Walking at 3 mph for 10 minutes  (1/6 hour), you cover 0.5 mile.

-- Average speed  =  (total distance covered) / (time to cover the distance) 

                                =        (10.5 miles) / ( 1/2 hour)

                                =            21 miles per hour. 

\(Formula = \ Average \ speed \ - \ (total \ distance \ covered) /\) \((time \ to \ cover \ the \ distance)\)

       \(Divide\)          \(=\ \textgreater \ (10.5 \ miles) / ( 1/2 \ hour)\)

                                          \(=\ \textgreater \ 21 \ miles \ per \ hour.\)

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