Correct Answer  Option 2 : 28
Calculation:
We can tabulate the given information as follows:

Time (in hours) 
Speed (in km/h) 
Initially 
t (assume) 
40 
Finally 
t + (6/60) 
35 
Hence, we obtain the distance as:
40t = 35 × [t + (6/60)]
⇒ 5t = 7/2
⇒ t = 42 minutes
So, the total distance = 40 × (42/60) = 28 km
onethirds of her usual time = 42/3 = 14 minutes
remaining distance = 28/3 km
To reach the office in the usual time, let the required speed be v km/h
⇒ (14/60) + (8/60) + (28/3v) = 42/60
∴ v = 28 km/h