Explanation:
First of all PLEASE PAY ATTENTION in class.
Secondly, I'll show you the method now.
We know that they paid 10$ for their parking and $90 altogether. So we can remove $10 from the $90. This now makes it $80.
We know that Michael, his wife and 2 other people (a couple) went to the zoo, that makes it 4 people total. We can use this number to divide with $80 making it $20. This therefore means that it was 20$ per person.
Here is my explanation in method form.
x =

x =

x = 20