Question 1182681
P((2nd fish is seabass) U (4th fish is catfish)) = 
P(2nd fish is seabass) + P(4th fish is catfish) -P((2nd fish is seabass) AND (4th fish is catfish)).
Now  P(2nd fish is seabass) = 21/80, while P(4th fish is catfish) = {{{(3/4)^3*(1/4) = 27/256}}}, since P(fish caught is not catfish) = 3/4 and P(fish caught is catfish) = 1/4.  
(Remember the non-catfish caught is thrown back to the pond until a catfish is caught).

==>P(2nd fish is seabass) + P(4th fish is catfish) - P((2nd fish is seabass) AND (4th fish is catfish)) = 21/80 + 27/256  - 1575/32000 = {{{highlight(51/160)}}}, or {{{highlight(0.31875)}}}.

***P((2nd fish is seabass) AND (4th fish is catfish)) = P(s,s,s,c) + P(s,s,t,c)
+ P(t,s,s,c) + P(t,s,t,c) = 1575/32,000.***