79. A train travels some distance with a speed of 20 km/h and returns with a speed of 40 km/h. Average speed of the train will be: a) 25km/h b) 26.67m/s 80. A body covers the first half clocity y and the second half in double the c) 30 m/s d) 60m/s
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36km/h36km/h
Solution
Let a distance xkmxkm be travelled with a speed of 30km/h30km/h.
Time taken , t1=x30ht1=x30h , On return journey , t2=x45ht2=x45h
Total time , t=t1+t2=x30+x45=x[130+145]t=t1+t2=x30+x45=x[130+145]
t=x(3+2)90=x18t=x(3+2)90=x18
Total distance =x+x=2x=x+x=2x
Average speed =total distancetotal time,=2xx/18=36km/h
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