CONCEPT: The question uses the application of Speed Time Concepts with Relative Speed as a major area of focus here.

The distance downstream is 90 miles and upstream too is 90 miles.
Hence if we divide 90 by 2.5 and 90 by 3(1/2 an hour more than 2.5) we have integer values  of 36 and 30 mph respectively  and we can choose that option.(A here)

Every other option(including the 1/2 hour addition) will lead to decimal values and it is relatively a safer option to go for under a time crunch.
Hope this helps.