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2015-04-23T23:36:56+05:30

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Distance D1 = 15 km     Speed V1 = 30 kmph      Duration T1 = 15/30 = 0.50 hrs
Distance D2 = 75 km     speed V2 = 50kmph       duration T2 = 75/50 = 1.5 hrs
Distance D3 = 10 km     speed V3 = 20 kmph      duration T3 = 10/20 = 0.5 hrs
 total      D = 100 km                                                      total  =  2 hours

   average speed = V = total distance / total time duration
                                 = 100 km / 2 hrs = 50 kmph

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We can also do this by using the formula:

V=\frac{total\ distance}{total\ time\ duration}\\\\.\ \ \ \ =\frac{D1+D2+D3}{T1+T2+T3}=\frac{D1+D2+D3}{\frac{D1}{V1}+\frac{D2}{V2}+\frac{D3}{V3}}\\\\V=\frac{15+75+10\ km}{0.50+1.50+0.50\ hrs}=\frac{100\ km}{2\ hrs}=50\ kmph

If you can use the formula, that is fine. Other wise do that in the first method.

2 3 2
I think your wrong.Because the answer is 40 km/h in the book back
as per the data in the question given, 50kmph is the answer. perhaps there is misprint in the key in the book.. you could verify yourself.
please click on thanks (azur blue button) above
why nt sir ?
when the number 15 is divided by 30 then the answer will be 0.5.But you put 0.50