Least common multiple of 3 and 5

Multiples of 3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30 ....

Multiples of 5: 5, 10, 15, 20, 25, 30 ...

3 and 5 both go into 15

3 and 5 both go into 30

3 and 5 both go into 60

15 is the least common multiple Geometric sequence - multiplication - sequence of numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio. For example, the sequence 2, 6, 18, 54, ... is a geometric progression with common ratio 3

- Wikipedia

Arithmetic progression (AP) or arithmetic sequence - addition - is a sequence of numbers such that the difference between the consecutive terms is constant. For instance, the sequence 5, 7, 9, 11, 13, 15, . . . is an arithmetic progression with common difference of 2

-Wikipedia Counting by multiplication, not addition (1, 2, 3...)

When there are m ways to do one thing,

and n ways to do another,

then there are m x n ways of doing both.

You have 2 pairs of shoes & 3 pairs of socks.

That means you have 2 x 3 = 6 combinations

White socks, black socks

White shoes, black shoes, brown shoes

White socks, white shoes

White socks, black shoes

White socks, brown shoes

Black socks, white shoes

Black socks, black shoes

Black socks, brown shoes

(If you tried to count by addition -- 1 + 1 + 1 + 1 + 1, or 1, 2, 3, 4, 5 -- you would get the wrong answer.)

Source: https://www.mathsisfun.com/data/basic-counting-principle.html