duck soup vs plain sailing

duck soup

noun

  • Something which is easy; a piece of cake. 

  • Used other than figuratively or idiomatically: see duck, soup. 

plain sailing

noun

  • Something that is simple, straightforward, or easy; something that offers no troubles or difficulties. 

  • The navigation of waters free from hazards or unfavourable winds. 

adj

  • simple, straightforward, or easy 

How often have the words duck soup and plain sailing occurred in a corpus of books? (source: Google Ngram Viewer )