That's because you can actually find the solution for any possible value of x y.
If x*y = 18 we can have six cases:
x = 1, y = 18
x = 2, y = 9
x = 3, y = 6
and the opposite.

Looking at the provided set of numbers then we see that in all the cases the biggest number is always 20 and the least is 1.
Thus the Range = 20 - 1 = 19.

Answer D.
Hope it is clear.

Let's examine the two extreme cases

Since x and y are positive integers, the SMALLEST difference between the two numbers occurs when x and y are 3 and 6.
Conversely, the GREATEST difference between the two numbers occurs when x and y are 1 and 18.

If x and y are 3 and 6, the set becomes {1, 6, 3, 17, 20, 6}, so the range = 20 - 1 = 19
If x and y are 1 and 18, the set becomes {1, 6, 1, 17, 20, 18}, so the range = 20 - 1 = 19

So the range is most definitely 19

Answer: D

Cheers,
Brent

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