Hi AleksandrM,
A relatively easy approach to this problem is to plug in numbers for X and Y.
For example, if X = 20 meters per second, then you know that equals 72,000 meters per hour (multiply 20 by 60 to get the meters per minute, and by another 60 to get the meters per hour), which translates to Y = 72 kilometers per hour.
Then, just plug in the numbers you used for X (= 20) to get the correct answer (Y = 72). You'll see that answer (C) is correct, since 18*20 = 360, and 360/5 is 72.
Note that it doesn't matter what you plug in for X, as long as you perform the calculations correctly.
Hope this helps!
Jim S. | GMAT Instructor | Veritas Prep
