Each entry came result in 4 exits.
There are 5 entries.
4 X 5 = 20
A football stadium has 5 gates on the eastern boundary & four gates on the western boundary a)In how many ways can a person enter by an east gate and leave by a west gate?
From the Internet---
The Fundamental Principle of Counting:
If something can be chosen, or can happen, or be done, in m different ways, and, after that has happened, something else can be chosen in n different ways, then the number of ways of choosing both of them is
m · n.