Since you've used the term "common ratio" it can be assumed that you're talking about a geometric sequence (each term is multiplied by some number -- the common ratio -- to get the next term in the sequence.
Simple example: 1, 3, 9, 27, 81, ... Each numbr is multiplied by 3 to get the next number in the sequence.
To get the common ratio, just divide a term by the term that comes immediately before it. In the example above, if you divide, say 27 by 9, you get the common ratio, 3.
In your problem, you could just divide 112 by 64 to get 1.75.
To check this, you can take any term, multiply it by 1.75 and get the following term: 112 x 1.75 = 196.