A waiter had some customers. After 9 customers left he still had 12 customers. How many customers did he have at the start?
The waiter has some customers, let's denote it with $C$

After 9 customers left, he still had $12$ customers, so the number of customers he had at the start was $C + 9 = 12$

We know that $C + 9 = 12$, so $C = 3$

The waiter had 3 customers at the start.