Question

Q.1
You drive 6.0 km at 50 km/h and then another 6.0 km at 90 km/h. Your average speed over the
12 km drive will be
A) greater than 70 km/h.
B) equal to 70 kmh.
C) less than 70 km h.
D) exactly 38 km/h.​

Answer 1

Given:

You drive 6.0 km at 50 km/h and then another 6.0 km at 90 km/h.

To find:

Average speed ?

Calculation:

• Let average speed be v :

$\rm \: v = \dfrac{total \: distance}{total \: time}$

$\rm \implies\: v = \dfrac{6 + 6}{ \dfrac{6}{50} + \dfrac{6}{90} }$

$\rm \implies\: v = \dfrac{12}{ \dfrac{6}{50} + \dfrac{6}{90} }$

$\rm \implies\: v = \dfrac{2}{ \dfrac{1}{50} + \dfrac{1}{90} }$

$\rm \implies\: v = \dfrac{2}{ \dfrac{9 + 5}{450} }$

$\rm \implies\: v = \dfrac{2}{ \dfrac{14}{450} }$

$\rm \implies\: v = 64.28 \: km {hr}^{ - 1}$

$\rm \implies\: v < 70 \: km {hr}^{ - 1}$

So, average speed is less than 70 km/hr.