One day, my mother and I went shopping. Mom went into the supermarket to buy things and let me stand at the place where I paid. I have nothing to do, just watch the assistant aunt collect money. After reading it, I suddenly found that the money collected by the assistant aunt was 1 yuan, 2 yuan, 5 yuan, 10 yuan, 20 yuan and 50 yuan. I feel very strange: Why isn't RMB from 3 yuan, 4 yuan, 6 yuan, 7 yuan, 8 yuan, 9 yuan or 30 yuan, 40 yuan or 60 yuan? I ran to ask my mother, and my mother encouraged me to say, "Think hard and calculate well. My mother believes you can figure out the reason." I calmed down and thought it over. After a while, I jumped up happily: "I know, because as long as you have 1 yuan, 2 yuan and 5 yuan, you can form 3 yuan, 4 yuan, 6 yuan, 7 yuan, 8 yuan and 9 yuan at will, and as long as you have 10 yuan, 20 yuan and 50 yuan, you can also form 30 yuan and 40 yuan." Why 2 yuan and 5 yuan? "I said," it is not convenient to use 1 yuan to form a larger number. "Now my mother showed a satisfied smile and praised me for observing more and thinking more. I am really more comfortable than eating my favorite ice cream.