Oh, send him to school... 3/5 means "three divided by five", which is less than one. 3.5 means "three items, plus one half of an item".
Casca Says, "For I Believe They Are Portentous Things/unto The Climate That They Point Upon." What Does He Mean?
Casca means "these things are no random, natural phenomena. Instead they're quite meaningful, they are portending (=prophesying) an ill omen for the land ("climate") over which they appear".