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