We KNOW that AVERAGE speed=TOTAL DISTANCE traveled/time taken.now distance=30+30=60km.and TIMES t=distance/speed.so t1=30/40=3/4hours.and t2=30/20=3/2hours.so total time=t1+t2=3/2 + 3/4=9/4.so average speed=60/(9/4)=240/9=26.66kmph
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