Factors are numbers we can multiply together to get another number.
Example: What are the factors of 12?
3 × 4 = 12
Also 2 × 6 = 12
And 1 × 12 = 12
So Factors of 12 are 1,2,3,4,6 and 12.
For finding factor of any number, just make sure you have thought about all possible combinations of getting that number.
When you find that the factors of two or more numbers are same or common, then they are the "common factors".
Coming to your question,
The factors of 20 are: 1, 2, 4, 5, 10, 20
The factors of 50 are: 1, 2, 5, 10, 25, 50
Common factors are 1, 2, 5, 10
Then the greatest common factor is 10.
The highest common factor of the four numbers is 1.
50 and 40's highest common factor is ten.
The least common factor of any set of integers is 1.
The GCF is 2.
There is no "common factor" of 350, to have a common factor you must have more than one number, so there is no common factor of 350.