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A truck go uphill at the rate of 20 km an hour and down hill at the rate of 40 km an hour. After 20 hours, it has covered 640 km. How long does if go down hill and uphill respectively? 

A truck go uphill at the rate of 20 km an hour and down hill at the rate of 40 km an hour. After 20 hours, it has covered 640 km. How long does if go down hill and uphill respectively?  Correct Answer (8, 12) hours

Given∶

Distance = 640 km

Speed on uphill = 20 km/hr

Speed on down hill = 40 km/hr

Time = 20 hours

Formula Used∶

Concept of Linear Equations, Distance = Speed × Time

Calculation∶

Let the time takes for uphill journey = x hrs

time takes for downhill journey = y hrs

∴ x + y = 20   

and 20x + 40y = 640  

Solving above equations, we get

20y = 240

⇒ y = 240/20 = 12 and

x + 12 = 20

⇒ x = 20 - 12 = 8

∴ Time taken for Uphill journey is 8 hours and  for Down hill journey is12 hours.

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