Given two crystal space groups, how can one determine if they are in a group-subgroup relationship? The specific case at hand is P 21/m 2/m 2/a (aka P m m a, #51) and P m m 2 (#25), but knowing how to solve this question in a general manner will sure prove helpful in the future as well…
The Pmma is a minimal non-isomorphic supergroup of Pmm2; or you can say that Pmm2 is one of maximal non-isomorphic sub-groups of Pmma.
How to solve the question:
(1) fast and dirty: take International Tables for Crystallography vol. A go to your group (in mine edition Pmma starts on p.274). On second page describing space group you can find Maximal non-isomorphic subgroups. The third listed is
 Pmm2 (25) 1; 2; 7; 8.
You can see that the operation in #50 and #25 have a lot of similarities. No.1 you can ignore as it's [x,y,z] but 2, 7 and 8 for #50 are [-x+.5,-y,z], [x,-y,z] and [-x+.5,y,z]. As one can see the #25 has following general operators: (2) [-x,-y,z] (3) [x,-y,z] and (4) [-x,y,z].
(2) The hard way: Mathematics is explain in the same volume in chapter 13.1. Isomorphic subgroups. Of course the knowledge of group theory and matrix calculus is to your advantage ;-)