When Devon cashed a $450 check at the bank, the teller gave him 18 bills, all $20 bills and $50 bills. Which system of equations can be used to find how many of each type of bill Devon received?
A) f- t = 18; 50f + 20t =450
B) f + t = 18; 50f + 20t = 450
C) f + t = 450; 50f + 20t = 18
D) f + t = 18; 50f - 20t =4 50