A frog sits on one of 4 lily pads. On each turn it jumps to one of the other 3 pads, each equally likely. After 4 jumps, what is the probability that the frog is back on the pad where it started?
一只青蛙停在 4 片荷叶中的一片上。每一步它等概率地跳到其余 3 片中的一片。4 步之后,它回到出发那片荷叶的概率是多少?