B. 35 minutes
Feedback Time taken to cover 24 Km at the speed of 8 Kmph = 24/8 = 3 hours
Thus, right time was 2 hours 35 mintes
Now, time taken to cover 20 Km at the speed as 12 Kmph = 2 hours
Required time = 2 hours 35 minutes - 2 hours
= 35 minutes