Look for the biggest number that they can all be divided by, and if there is a variable see if they can all have at least one of that variable taken away, so for example, in the numbers 20, 50, and 65 the greatest common factor is 5, there is no variable so you could just say 5 (4 + 10 + 13), however let’s say you came across 20y + 50y^4 + 65y^2, they all have at least one variable to take from so in this case you would factor using 5y, and it would look like this 5y (4 + 50y^3 + 65y). I hope that made sense, please tell me if you need further explanation and I’ll try my best to put it in an easier or more descriptive way.