Kerry asked a bank teller to cash 390 check using 20 bills and 50 bills. If the teller gave her a to

Kerry asked a bank teller to cash 390 check using 20 bills and 50 bills. If the teller gave her a total of 15 bills, how many of each type of bill did she receive?

Let t = number of 20 bills and f = number of 50 bills. We have two equations.
(1) 20t + 50f = 390
(2) t + f = 15

Rearrange (2) into (3) for t, by subtracting f from each side:
(3) t = 15 - f

Now substitute (3) into (1)
20(15 - f) + 50f = 390
300 - 20f + 50f = 390

Combine f terms
300 + 30f = 390

Subtract 300 from each side
30f = 90

Divide each side by 30
f = 3

Substitute f = 3 into (3)
t = 15 - 3
t = 12