Moving at the rate of 70 km/hr, a car completes a journey in 18 minutes. How long would it take to complete this journey, if the speed is increased to 84 km/hr?

Time taken at 70 km/hr speed = 18 minutes.

Time taken at 1 km/hr speed = (18 x 70) minutes. $\quad$ [Less speed, More time]

Time taken at 84 km/hr speed = $\Big(\dfrac{18 \times 70}{84}\Big) \text{ minutes} \quad \text{[More speed, Less time]}$

= $\Big(\dfrac{3 \times 70}{14}\Big) \text{ minutes} \quad \text{[Dividing 18 and 84 by 6]}$

= 3 x 5 minutes $\quad$ [Dividing 70 and 14 by 14]

= 15 minutes.

At the rate of 84 km/hr, the car completes the journey in 15 minutes.