A waiter had some customers. After 9 customers left he still had 12 customers. How many customers did he have at the start?
The number of customers at the start is $x$. The number of customers at the end of the first week is $x-4$. The number of customers at the end of the second week is $x-4-3=x-7$. The number of customers at the end of the third week is $x-7-2=x-9$. Finally, the number of customers at the end of the fourth week is $x-9-1=x-10$.

Thus, we have the equation $x-10=0$, from which it follows that $x=10$.