ANSWER
20 + 5.50x = 157.50
Step-by-step explanation
We have that Antonio is paid $20.00 per day plus $5.50 per table served.
Let the number of tables served in a day be x.
It means that if he serves x tables, he will get paid:
5.50 * x = $5.50x
On Friday, he earns $157.50.
This means that if we add his daily pay ($20.00) to his pay for x tables ($5.50x), we will have $157.50.
That is:
20 + 5.50x = 157.50
That is the equation that can be used to determine how many tables he served on Friday.