That was a “Piece of Cake”
Referring to something as a "piece of cake" is often used to describe a situation that was easy, or required little effort. How did the term cake come to mean easy?
The idea of cake being “easy” originated in the 1870’s when cakes were given out as prizes for winning competitions. In particular, there was a tradition in the US slavery states where slaves would circle around a cake at a gathering. The most “graceful” pair would win the cake the in middle. From this the term “cake walk” and “piece of cake” came into being, both meaning that something was easy to accomplish.
The term was first used in print in 1936 by Ogden Nash who wrote, “Her picture’s in the papers now, and life’s a piece of cake.” Today the term is commonly used to describe tests at school, or friendly competitions.
Information from: BusinessBalls.com and The Phrase Finder