Let the number of $10 bills be x Let the number of $20 bills be y Value Value Type Number of of of of EACH ALL bill bills bill bills ------------------------------------------- $10 bills x $10 $10x $20 bills y $20 $20y ------------------------------------------- TOTALS 16 ----- $220 The first equation comes from the second column.x + y = 16 The second equation comes from the last column. 10x + 20y = 220 So we have the system of equations: . We solve by substitution. Solve the first equation for y: x + y = 16 y = 16 - x Substitute (16 - x) for y in 10x + 20y = 220 10x + 20(16 - x) = 220 10x + 320 - 20x = 220 -10x + 320 = 220 -10x = -100 x = 10 = the number of $10 bills. Substitute in y = 16 - x y = 16 - (10) y = 6 $20 bills. Checking: 10 $10 bills is $100 and 6 $20 bills is $120 That's 16 bills. And indeed $100 + $120 = $220 Edwin