a piece of cake: 누워서 떡 먹기

A: How was the test?
   - 시험 어땠어?

B: It was a piece of cake. I finished it first.
  - 누워서 떡먹기였어. 내가 제일먼저 끝냈지.

-> It was very easy! 너무 쉬웠어. Icing on the cake 금상첨화/