How is 20//6=3??

Answer 1

// is floor division.
20/6 is something in decimal *not good in math*
and 20//6 is 3 as 6*3=18

Basically it gives integer value.

Answer 2

// is integer division, which floors the result and returns how much y's fully fit into x for x // y.
If you want real division, use single / instead, but remember it returns a float.