You can always do it though sometimes it creates larger numbers than you may want to work with. One way of thinking about it is to take the 2 numbers (from top and bottom) and pretend they are denominators and find the common denominator then multiple each by what it needs to become the common denominator.With 4 and 5 the common denominator would be 20, the same as it you just multiplied them together but what if you had the problem:4x - 5y = 2510x + 3y = 15you can multiply the top by 10 and the bottom by 4 or find the "common denominator" for 4 and 10 which is 20. The top would then be multiplied by 5 and the bottom by 2. Smaller numbers to work with. Both ways will get you the correct answer.