Question 791282
when the speed is 40 km/hr, we know this much


d = rt


d = 40t


t = d/40


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when the speed is 60 km/hr, we know this much


d = rt


d = 60t


t = d/60


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So it takes d/40 hours when the speed is 40 km/hr and it takes d/60 hours when the speed is 60 km/hr



Add up the two times



d/40 + d/60 = 3d/120 + 2d/120


d/40 + d/60 = 5d/120


d/40 + d/60 = d/24



So overall, it will take d/24 hours (for the entire round trip)



For the sake of simplicity, let's say d = 24 (so that d/24 turns into 24/24 = 1)


Because going one way will have you going 24 km, this means that completing a full round trip will have you traveling 2*24 = 48 km


So because you have traveled 48 km in one hour (this is average over the entire trip), the average speed is <font color="red">48 km/hr</font>