Basicly you have to find out what the pattern is... Are you sure the question was 25__61 because I don't think you can find a number out for that one unless it was a .5 answer or something simular but say like ... 20 ___ 40___ 60 ... Youd no because its the middle number like inbetween 20 and 40 is 10 so its +10... If it was like 20___60___140 it would be + 20 because 20 + 20 = 40 so youve added 20 onto the 20 to get 40 and if you was to add 20 onto the sixty youd gett 100 it's quite simple ... But it depends what the number sequence is because those 2 I have just said about are simple ones

I meen, if you added 20 onto 20 it would be 40 and if you added 20 onto 40 it would be sixty so youd no the pattern goes up in 20's

90, 30, 180, ____, 360, 120

81 27 9

25 36 61 72

4 12 6 12 36 18 36