AS Bio Help

The enzyme RNAase is a protein containing 124 amino acids.

a) How many different sorts of molecules can be made containing 124 amino acids, assuming that there are 20 different amino acids involved in protein structure?

b)In what ways do the two ends of the RNAase molecule differ from each other, apart from involving different amino acids?

c)Why do all molecules of RNAase have the same tertiary structure?

Any help please?