Let us say the total distance is x.
Distance covered during first hour =0.25x
Distance covered during second hour =0.3 x
Distance covered during third hour =0.2x
Total distance covered after three hours will be sum of all the three distances.
Distance covered after three hours = 0.25x +0.2x +0.3x = 0.75x
So distance covered after three hours is 0.75 times of total distance.
Distance left to be completed = total distance - distance covered in three hours
Distance left = x- 0.75 x = 0.25x
So distance of trip left to be completed is 0.25 times total distance.