Basically a simple proportion/rate/ratio
6 : 51 = 3 : ?
If you don't understand this is that as an analogy in english:
6 is to 3 as 51 is to ?
Well? 3 is half of 6 so whatever ? is, it's half of 51!
51/2 =
25.5
Hope that helps and correct me if I'm wrong!