Question 15 Marks

A car travels with speed $30 \ km\  h^{-1}$ for $30$ minute and then with speed $40 \ km\  h^{-1}$ for one hour.
Find $:$
$(a)$ the total distance travelled by the car
$(b)$ the total time of travel, and
$(c)$ the average speed of car

Answer

Speed of car for first $30$ minutes $=30 \ km \ h ^{-1}$
Speed of car for next $1$ hour $=40 \ km\ h ^{-1}$
$(a)$ Total distance travelled by the car
$1^{st}$ case , Speed $=\frac{Distance}{Time}$
$\Rightarrow$ Distance $=$ Speed $\times$ Time
$(\because 30$ minutes $=0.5$ hours$)$
Distance $=30 \times 0.5$
$\quad=15 \ km \ldots \ldots . . .(1)$
IInd case Speed $=\frac{Distance}{Time}$
$\Rightarrow$ Distance $=$ Speed $\times$ Time
Distance $=40 \ km \ h ^{-1} \times 1\ hr$
$=40 \ km$
Adding $(1)$ and $(2)$
Total Distance $=15 \ km +40 \ km =55 \ km$
$(b)$ Total time of travel $=0.5\ hr +1.0\ hrs =1.5\ hr$
$(c)$ Average speed $=\frac{\text { Total distance travelled }}{\text { Total time taken }}$
$=\frac{5.5 \ km }{1.5\ hr }\ [$Form above $(a)$ and $(b)]$
$=36.67 \ km\ h ^{-1}$

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