Solve pattern 66 73 13 21 52 _ 10 20

The answer would be 61. First, if you are trying to figure out the pattern. You must subtract the second number with the first then fourth number with the fifth and so on, you get.

73 minus 66 equals 7.
21 minus 13 equals 8.
XX minus 52 equals x.
20 minus 10 equals 10.

Basing on the pattern of the set of numbers, x will equal 9, To get this you must do 52 and 9 which will equal 61.