The pattern is going backwards, and every time, the gap between each number increases by one.
For example 49 comes directly after 48 when counting, so in this pattern, 48 is one behind 49. While 46 is two behind 48, and so on and so forth.
49 - 48 - - 46 - - - 43 - - - - 39 - - - - - 34 - - - - - - 28 - - - - - - - 21.
Hope that makes sense.
