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A car travelled at 40 km/h for the first 2 hours. It travelled at 60 km/h for the last 3 hours. What is its average speed?
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\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{40\times 2 + 60\times 3}{2+3} \\ &= \frac{260}{5} \\ &= \frac{52\text{ km}}{h} \end{align*} Common mistake: $\dfrac{40+60}{2} = \dfrac{50\text{ km}}{h}$
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Charles was walking at 10 km/h for the first 3 hours. Later Charles was walking at 20 km/h for the last 7 hours. What is Charles's average speed?
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 3 + 20\times 7}{3 + 7} \\ &= \frac{170}{10} \\ &= \frac{17\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+20}{2} = \dfrac{15\text{ km}}{h}$
A slow train travelled at 10 km/h for the first 2 hours. It travelled at 30 km/h for the last 3 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 2 + 30\times 3}{2 + 3} \\ &= \frac{110}{5} \\ &= \frac{22\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+30}{2} = \dfrac{20\text{ km}}{h}$
A horse ran at 10 km/h for the first 2 hours. It ran at 30 km/h for the last 8 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 2 + 30\times 8}{2 + 8} \\ &= \frac{260}{10} \\ &= \frac{26\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+30}{2} = \dfrac{20\text{ km}}{h}$
A car travelled at 10 km/h for the first 3 hours. It travelled at 30 km/h for the last 7 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 3 + 30\times 7}{3 + 7} \\ &= \frac{240}{10} \\ &= \frac{24\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+30}{2} = \dfrac{20\text{ km}}{h}$
A truck travelled at 10 km/h for the first 6 hours. It travelled at 30 km/h for the last 9 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 6 + 30\times 9}{6 + 9} \\ &= \frac{330}{15} \\ &= \frac{22\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+30}{2} = \dfrac{20\text{ km}}{h}$
A train travelled at 10 km/h for the first 3 hours. It travelled at 40 km/h for the last 7 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 3 + 40\times 7}{3 + 7} \\ &= \frac{310}{10} \\ &= \frac{31\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+40}{2} = \dfrac{25\text{ km}}{h}$
A dragon travelled at 10 km/h for the first 2 hours. It travelled at 50 km/h for the last 3 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 2 + 50\times 3}{2 + 3} \\ &= \frac{170}{5} \\ &= \frac{34\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+50}{2} = \dfrac{30\text{ km}}{h}$
A shark travelled at 10 km/h for the first 2 hours. It travelled at 50 km/h for the last 6 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 2 + 50\times 6}{2 + 6} \\ &= \frac{320}{8} \\ &= \frac{40\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+50}{2} = \dfrac{30\text{ km}}{h}$
A bird travelled at 10 km/h for the first 2 hours. It travelled at 50 km/h for the last 8 hours. What is its average speed?'
\begin{align*} \text{Average speed} &= \frac{\text{Total Distance}}{\text{Total Time}} \\ &= \frac{10\times 2 + 50\times 8}{2 + 8} \\ &= \frac{420}{10} \\ &= \frac{42\text{ km}}{h} \end{align*} Common mistake: $\dfrac{10+50}{2} = \dfrac{30\text{ km}}{h}$
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