### Theory:

Common factor
When we find factors for two or more numbers if any factors are common (same) between the numbers, they are called common factors.
Example:
1. Find the common factors of $$8$$ and $$24$$.

 $$1 \times 8 = 8$$ $$2 \times 4 = 8$$ $$4 \times 2 = 8$$

We stop here because $$2$$ and $$4$$ already exist as a factor.

 $$1 \times 24 = 24$$ $$2 \times 12 = 24$$ $$3 \times 8 = 24$$ $$4 \times 6 = 24$$ $$6 \times 4 = 24$$

We stop here because $$4$$ and $$6$$ already exist as a factor.

Common factors of $$8$$ and $$24$$ are $$1$$, $$2$$, $$4$$ and $$8$$.

2. Find common factors of $$15$$, $$45$$ and $$50$$.

 $$1 \times 15 = 15$$ $$3 \times 5 = 15$$ $$5 \times 3 = 15$$

We stop here because $$3$$ and $$5$$ already exist as a factor.

 $$1 \times 45 = 45$$ $$3 \times 15 = 45$$ $$5 \times 9 = 45$$ $$9 \times 5 = 45$$

We stop here because $$5$$ and $$9$$ already exist as a factor.

 $$1 \times 50 = 50$$ $$2 \times 25 = 50$$ $$5 \times 10 = 50$$ $$10 \times 5 = 50$$

We stop here because $$5$$ and $$10$$ already exist as a factor.

Common factors of $$15$$, $$45$$ and $$50$$ are $$1$$ and $$5$$.