Math problem for the second grade: How many kinds of things with different values can you buy with a 5 yuan, a 10 yuan and a 1 RMB?
With one, there are three kinds;
Use two:
5+ 10= 15
5+ 1=6
10+ 1= 1 1
There are also three kinds;
Use all three:
5+ 10+ 1= 16
There is a kind of
So * * * has: 3+3+ 1=7 (species)