To get to work, you drive for 20 minutes at 30 mph and walk 10 more minutes at 3 mph what was your average speed?

-- 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.