Question

A particle goes from $A$ to $B$ with a speed of $40kmh^{- 1}$ and $B$ to $C$ with a speed of $60kmh^{- 1}$ . If $AB=6 \, BC$ , the average speed in $kmh^{- 1}$ between $A$ and $C$ is

Answer 1

$V_{av}=\frac{\text{distance travelled}}{\text{time taken}}$
$\Rightarrow V_{av}=\frac{A B + B C}{\frac{A B}{40} + \frac{B C}{60}}$
$\Rightarrow V_{av}=\frac{6 B C + B C}{\frac{6 B C}{40} + \frac{B C}{60}}$
$V_{av}=\frac{7 \times 2 4 0}{3 6 + 4}\Rightarrow V_{av}=\frac{7 \times 2 4 0}{4 0}$
$\Rightarrow V_{av}=42km/h$