One third part of a certain journey is covered at the speed of 24 km/hr, one fourth part at the speed of 36 km/hr and the rest part at the speed of 60 km/hr. What will be the average speed for the whole journey?

Correct option is B

​Given:
One-third of the journey is covered at a speed of 24 km/hr.
One-fourth of the journey is covered at a speed of 36 km/hr.
The remaining part of the journey is covered at a speed of 60 km/hr.
Total distance for the journey = 12 km.
Formula Used:
​$$\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}\\\ \\\text{Total Time for each part} = \frac{\text{Distance}}{\text{Speed}}$$​​
Solution:
Distance covered at 24 km/hr =$$\frac{1}{3} \times 12 = 4km$$​​
Distance covered at 36 km/hr = $$\frac{1}{4} \times 12 = 3km$$​​
Remaining distance =12 - (4 + 3) = 5km
Time for first part $$=\frac{4}{24} = \frac{1}{6}hours$$​​
Time for second part =$$\frac{3}{36} = \frac{1}{12}hours$$​​
Time for third part =$$\frac{5}{60} = \frac{1}{12}hours$$​​
​$$\text{Total Time} = \frac{1}{6} + \frac{1}{12} + \frac{1}{12} = \frac{1}{3} \text{ hours}$$​​
​$$\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}} = \frac{12}{\frac{1}{3}} = 12 \times 3 = 36 \text{ km/hr}$$​​