Explain how to solve this math problem: Max has a collection of stamps in three colors: 20 red stamps, 80 blue stamps and 7 yellow ones. He is trying to sell the whole collection. He already sold 20 red stamps for the price of $1.1 for each stamp and 80 blue stamps for the price of $0.8 for each stamp. What price did he need to set for each yellow stamp to be able to earn the total of $100 from the whole sale?