Coin and Stamp Problems

Solving Coin or Stamp Problems using 1 variable: 

Coin Word Problem 1: http://www.youtube.com/watch?v=NkI-WP2vosk

Coin Word Problem 2: http://www.youtube.com/watch?v=rQuWBFLjGR0

Coin Word Problem 3: http://www.youtube.com/watch?v=bwsoDEzCpjo

Stamp Word Problem 1: http://www.youtube.com/watch?v=erP7aJawVlc

Solving Coin or Stamp Problems using 2 variables: 

Stamp Word Problem - 2 variable systems of equations: Juan has 80 stamps. Some were 29¢ stamps and some were 45¢ stamps. The total value of all the stamps was $27.04. How many of each type of stamp does he have?