Question

How many times would you expect proline to appear in this peptide if it were made up of a completely random assortment of the 20 most common amino acids?

Answer 1

Out of the 20 most common amino acids, proline appears with a frequency of about 5%.
So, if a peptide were to be made up of a completely random assortment of the 20 most common amino acids, proline would appear about 5 times in 100 amino acids.