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2015-07-23T17:15:28+05:30

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Let the total distance to cover = d km

one third of the distance is covered with 20 kmph
So d₁ = d/3
v₁ = 20 km/h
t₁ = d₁/v₁ = (d/3)/20 = d/(3*20) = d/60 hour

Rest distance, d₂ = 2d/3
v₂ = 60 kmph
t₂ = d₂/v₂ = (2d/3) / 60 = d/90 hour

total time,t = t₁ + t₂ = d/60 + d/90 = 5d/180 = d/36 hour

average speed = d/t = d/(d/36) = 36 km/h
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2015-07-28T12:24:32+05:30

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
LET TOTAL DISTANCE = X km  

X1 = X/3 km  
V1 = 20 km/h  
X2 = 2X/3 km  
V2 = 60 km/h  

SPEED = DISTANCE / TIME  

TIME = DISTANCE / SPEED  

t1 = X1 / V1
t1  = X / 60 h  

t2 = X2 / V2
t2 = X / 90 h    

TOTAL TIME,  
 

t = t1 + t2
 
t = X / 60 + X / 90 = X / 36 h

AVERAGE SPEED = DISTANCE / TIME                                  
                              =  X / (X/36)
                         
                              =
 36 km/h
   
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