TO FACTOR ANY EXPRESSION, you would need to find the common factor between the numbers in the expression that are to be factored. In this case, you're factoring 20 and 50.
All you have to do is find a common factor between them, any common factor. It can be 5, it can be 2, it can be 10, it can be whatever goes into both numbers once multiplied.
If you were to use 2 to factor it, it would be : 2( 10k + 25)
If you were to use 5 to factor it, it would be : 5( 4k + 10)
And the greatest factor is 10, and it would be : 10( 2k + 5)