Question

Abdul, while driving to school,computes the average speed for his trip to be 20 km/h.on his return trip along the same route,there is less traffic and the average speed is 40km/h.what is the average speed for abdul's trip?

Answer 1

ANSWER
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$let \: the \: distance \: between \: home \: and \: school \: be \: s\\ \\ average \: speed \: for \: home \: to \:school \: = \: 24km {h}^{ - 1} \\ \\ speed \: = \frac{distance}{time} \\ \\ 20 = \frac{s}{t} \\ \\ t \: = \: \frac{s}{20} h \\ \\ average \: speed \: for \: school \: to \: home \: = \: 40km {h}^{ - 1} \\ \\ speed \: = > \: 40 = \frac{s}{t} \\ \\ t \: = \frac{s}{40} \\ \\ total \: distance \: travelled \: = s + s \: = \: 2s \\ \\ average \: speed \: = \: \frac{total \: distance \: travelled}{total \: \: time } \\ \\ = \frac{s \: + s}{ \frac{s}{20} + \frac{s}{40} } \\ \\ = \frac{2s}{ \frac{3s}{40} } \\ \\ = 2s \: \times \frac{40}{3s} \\ \\ = \: 26.666 \: km {h}^{ - 1} \\ \\ = 26.67 \: km {h}^{ - 1} \: \: \: \: ans$


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#BE BRAINLY

Answer 2

Answer:

24km/h

Explanation:

Given :

Method I:  

Let the distance traveled by Abdul from home to school = s km

time taken to reach the school=t1 sec

For Return journey , Abdul cover distance =s km

time=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/t2

t2=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/hr

Shortcut 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