as easy as falling off a log 易如反掌；非常容易；极容易的

【例句】
Why not assemble the kit yourself? It's as easy as falling off a log.
你为什么不自己把那套元器件装配起来，那实在太容易。

Roger can solve quadratic equations as easy as falling off a log.
罗杰能轻而易举地解二次方程式。