If you're looking for the number of dogs (I'm assuming, you didn't specify), all you'd have to do is divide the number by two, then round down to find the number of dogs, then check your answer by adding that number and that number plus 5.
For example, yours would look like 125/6=62.5, and rounding down to 60.
Then check your answer by adding 60 and 65, the number of cats. 60+65=125.
