A bowl contains three red (R) balls and seven white (W) balls of exactly the same size and shape. Select balls successively at random and with replacement so that the events of white on the first trial, white on the second, and so on, can be assumed to be independent. In four trials, make certain assumptions and compute the probabilities of the following ordered sequences: 

(a) WWRW

(b) RWWW

(c) WWWR

(d) WRWW. 

Compute the probability of exactly one red ball in the four trials.