A factor is one part of a product.
In the given expression first we separate the terms.
here we have two terms: 15 and 20x
factors of first term 15 : since just one number is there so factors of 15 would be 1, 3, 5 and 15
factors of second term 20x: product of 20 and x gives 20x, so factors of 20x are 1,2,4,5, 10,20 and x
