Question

A train travels 20km at a uniform speed of 60km/hr and the next 20km at a uniform speed of 80km/hr. Calculate its average speed.

Answer 1

Hello there !
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Average speed = total distance travelled / total time taken 
                         = 20 + 20  / 20/60 + 20/ 80 
                         = 40 /   80 + 60 /240
                         = 40 / 140 / 240 
                         = 40 X 240 / 140 
                         = 68.5 km/hr 
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Answer 2

hi there !

Average speed = total distance traveled / total time taken 

total distance traveled = 40 km
total time = 20/60 + 20/80
                = 1/3 + 1/4
                =  4 + 3 / 12 
                = 7/12 hr 

                        
average speed = 40 / [ 7/12 ]
                        = 40 * 12/7
                        = 68.571 km/hr