What is the greatest common factor of 63?

Answer 1

You need at least 2 numbers to find the GCF of a number.

The greatest common factor (GCF) is often also called the greatest common divisor (GCD) or highest common factor (HCF). Keep in mind that these different terms all refer to the same thing: the largest integer which evenly divides two or more numbers.

There cannot be a greatest common factor of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.

Answer 2

The letter "c" in "gcf" stands for the word "common".

"Common" means "same for all".

There can't be anything common about one single item.
It takes two of them, at least.