Hi Elnur1997,
This is a reference to TESTing VALUES, which is a really useful Tactic in the Quant section of the GMAT. Since "X" refers to the percent of the distance traveled at 40 miles/hour (with the remaining distance traveled at 60 miles/hour), using X = 50 is an easy way to 'split' the distance into 2 pieces.
Based on the way that the overall question is written, there's a really easy way to answer the question with this same strategic approach. Make sure to note what the question is asking for: we're asked, in terms of X, what was Francine's AVERAGE SPEED was for the entire trip.
IF....
Total Distance = 100 miles and X = 40....
40% of 100 = 40 miles, so Francine traveled 40 miles at 40 miles/hour --> 1 hour traveled
The rest = 60 miles, so Francine traveled 60 miles at 60 miles/hour --> 1 hour traveled
Total Distance traveled = 100 miles
Total Time traveled = 2 hours
Average Speed = 100/2 = 50 miles/hour
So, we're looking for an answer that equals 50 when X = 40. There's only one answer that matches...
GMAT assassins aren't born, they're made,
Rich