During a certain trip, Matt traveled y percent of the total distance

No calculations needed here if you understand weighted average and average speeds.

Step I
1/3rd distance at 40 mph and 2/3rd distance at 60 mph. So we can expect the average speed of these two to be somewhat above 50. (If equal distances were travelled at the two speeds, the average speed would have been a bit less than 50).

Step II
Now, depending on y, we will get an exact value for average speed which must lie between 48 mph and this speed which is above 50.
The minimum possible value we have in this range in the options is 50. If 52 mph is a possible option, then 50 definitely is (we will adjust the value of y accordingly). Since 50 is, 52 or higher must not be possible because avg speed in Step I must be less than 52.

Hence, answer can be nothing but (A).

Check: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2015/0 ... -the-gmat/

Where do you find y=40%?

How you consider distance as 600 miles?Is there any calculation process for considering 600 miles, i mean LCM or any other procedure?

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