Question

By increasing the speed of the car by $10km/hr$ the time of journey for a distance of $72km$ is reduced by $36$ minutes. Find the original speed of the car.

• A. $30km/hr$
• B. $40km/hr$
• C. $20km/hr$
• D. $50km/hr$

Answer 1

The correct option is A $30km/hr$

Let the original speed of the car $=xkm/hr$
$\therefore$ Time taken by it to cover $72km=\frac{72}{x}hrs$
New speed of the car = $(x+10)km/hr$
$\therefore$ New time taken by the car to cover $72km=\frac{72}{x+10}hrs$
As per the given condition, time is reduced by $36$ minutes
$\therefore \frac{72}{x}-\frac{72}{x+10}=\frac{36}{60}$
$x^2+10x-1200=0$

$(x-30)(x+40)=0$

On solving we get, $x=-40$ or $x=30$
Since the speed cannot be negative, so $x=30km/hr$.