a piece of cake 小事一桩，易如反掌，小菜一碟（常用于安慰别人）

【例句】
Getting rid of him will be a piece of cake.
摆脱掉他就是小事一桩。

This test was a piece of cake. I didn't have to study so much!
这次考试是小菜一碟，我用不着学那么多！