Question

a car cover the first half of the distance between two places at a speed of 40 km per hour and the second half 60 kilometre per hour what is the average speed
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Answer 1

(1) let x/2 distance cover in time t1 & another x/2 distance cover in time t2
t1 = (x/2)/40
t2 = (x/2)/60
t1+t2 = x/48
average speed = x/(t1+t2) 
                     = x/(x/48)
                     = 48Km/hr
(2) as the above
t1 = x/40
t2 = x/60
t1+t2 = x/24
average speed = 2x/(x/24)
                      =48Km/hr 
(3) s1 = 6*2 
           = 12Km.
s2 = 4*3
      = 12Km.
total distance = s1+s2 = 24km.
average speed = 24/5
                      = 4.8Km/hr