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3. Abdul, while driving to school, computes the average speed forhis trip to be 20 km h'. On his return trip along the sameroute, there is less traffic and the average speed is30 km h'. What is the average speed for Abdul's trip? |
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Answer» Let the distance traveled by Abdul from home to school = s kmtime taken to reach the school=t1 secFor Return journey, Abdul cover distance =s kmtime=t2 sec∴ Average speed for forward journey[home - school ] = Total distance/ Total time.20 km/h =s/t1∴t1=s/20 h ------eq(1)Average speed for backward journey[school -home] = Total distance/ Total time.30 km/h =s/t2t2=s/30h -----eq(2)Average distance for entire journey = Total distance/ Total time =(s+s)/[s/20 +s/30] =2s/s[1/20+1/30] =2x20x30/50 =24 km/hrShortcut method : If equal distance are covered with speeds v1 and v2 then average distance=2v1v2/v1+v2 = 2x20x30/50 =24km/hr ∴ The average speed for Abdul's trip is 24km/h |
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