A train covers the first 16 km at a speed of 20 km per hour another 20 km at 40 km per hour and the last 10 km at 15 km per hour. Find the average speed for the entire journey.

A. 24 km

B. 26 km

C. 21 km

D. $23 \frac{23}{59} \mathrm{~km}$

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Solution(By Apex Team)

$\begin{array}{l}\text{Average speed}\\ =\left(\Large\frac{\text{total distance covered}}{\text{total time}}\right)\\ \text{Total distance}\\ =16+20+10\\ =46\ \text{km}\\ \text{Time taken}\\ =\Large\frac{16}{20}+\frac{20}{40}+\frac{10}{15}\\ =\Large\frac{4}{5}+\frac{1}{2}+\frac{2}{3}\\ =\Large\frac{24+15+20}{30}\\ =\Large\frac{59}{30}\\ \text{ Average speed }\\ =\Large\frac{46\times30}{59}\\ =23\Large\frac{23}{59}\mathrm{~km}/\mathrm{hr}\end{array}$

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