Question 378064
Let a,b be the number of $10 and $20 bills, respectively. We have the system of equations

10a + 20b = 330
a + b = 20 (since there are 20 bills)

We can multiply the second equation by 10 to obtain 10a + 10b = 200. Substituting into the first equation we have

(10a + 10b) + 10b = 330
200 + 10b = 330
b = 13, a = 7

So the cashier has seven $10 bills, 13 $20 bills.