Question
A car travels a certain distance with $42 \mathrm{~km} \mathrm{~h}^{-1}$ for 20 minutes and the remaining distance at $60 \mathrm{~km} \mathrm{~h}^{-1}$ for 30 minutes. What is its average speed of the whole journey?
Step 1
We know that 60 minutes is equal to 1 hour. So, for 20 minutes the conversion factor would be $\frac{20}{60} = \frac{1}{3}$ hour and for 30 minutes, it would be $\frac{30}{60} = \frac{1}{2}$ hour. Show more…
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Let the distance travelled by car with $42 \mathrm{~km} \mathrm{~h}^{-1}$ for 20 minutes be $\mathrm{S}$, and distance travelled with 60 $\mathrm{km} \mathrm{h}^{-1}$ for 30 minutes be $\mathrm{S}_{2}$ $\Rightarrow \mathrm{S}_{1}=42 \mathrm{~km} \mathrm{~h}^{-1} \times \frac{1}{3} \mathrm{~h}=14 \mathrm{~km}$ $\Rightarrow \mathrm{S}_{2}=60 \mathrm{~km} \mathrm{~h}^{-1} \times \frac{1}{2} \mathrm{~h}=30 \mathrm{~km}$ Total distance $\mathrm{S}=\mathrm{S}_{1}+\mathrm{S}_{2}=14+30=44 \mathrm{~km}$
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A car travels a straight road at $100 \mathrm{~km} / \mathrm{h}$ for 30 min then at $60 \mathrm{~km} / \mathrm{h}$ for $10 \mathrm{~min}$. It then reverses and goes at $80 \mathrm{~km} / \mathrm{h}$ for 20 min. Find the average velocity and average speed for the entire trip.
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