Three-fifth of a journey is covered at a speed of 84 km/hr, and the remaining distance is covered at the rate of 56 km/hr. The average speed for the whole journey (in km/hr) is:

Correct option is A

Given:
Three-fifth of a journey is covered at a speed of 84 km/hr
Remaining distance is covered at the rate of 56 km/hr
Formula Used:
​$$\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}$$​​
Solution:
Let the total distance be 5km
So, Distance covered at a speed of 84 km/hr = $$\frac{3}{5} \times 5D = 3D$$ 
​Distance covered at a speed of 56 km/hr = $$5D-3D = 2D$$ 
Time taken to cover 3D distance at speed of 84 km/hr = $$\frac{3D}{84}$$  
Time taken to cover 2D distance at speed of 84 km/hr = $$\frac{2D}{56}$$ 
Average Speed = $$\frac{5D}{\frac{3D}{84} +\frac{2D}{56}} = \frac{5D}{\frac{6D+6D}{168}} = \frac{5D}{\frac{12D}{168}} = 70 km/hr$$ ​