Hi,

Note D = r*t   20min = 1/3 hr and 10min = 1/6 hr

(1/3hr)30mph + (1/6hr)3mph = 10.5 mi, the total distance traveled.

 total distance / total time = average speed = 10.5/(1/3 + 1/6) = 10.5/(2/6 + 1/6)

10.5mi/.5hr = 21mph 

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