What is the approximate probability of exactly one default over the next year froma portfolio of 20 BBB-rated obligors? (Assume the 1-year probability of default fora BBB-rated counterparty to be 4%...

What is the approximate probability of exactly one default over the next year froma portfolio of 20 BBB-rated obligors? (Assume the 1-year probability of default fora BBB-rated counterparty to be 4% and obligor defaults to be independent fromone another)
Standard and Poor’s rating agency have compiled actual default rates using data over the last twenty years, what are their approximate computed odds for the default of a CCC rated bond over a five year period?