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average speed=total distance/time taken ; distance=speed x timeFor the first part of her motion the distance she travelled, d_{1}=40 x 1/2 (as 30 minutes represent half an hour), so d_{1}=20 milesFor the second part the distance she travells is 0 because she stops for 15 minutes.For the third part: d_{3}=60 x 1/4 (as 15 minutes represent a quarter of an hour), so d_{3}=15 miles.average speed=(20+0+15)/1=35 miles per hour