Lets say, we put all the red marbles in jar A and all blue marbles in jar B. Then the probability of getting a red marble is :

jar B : (1/2)*0 = 0 (selecting the jar B = 1/2, red marble from jar B = o/50)

So the maximum probability will be :

jar A : (1/2)*1 = 1/2 (selecting the jar A = 1/2, red marble from jar A = 1/1)

jar B : (1/2)*(49/99) = 0 (selecting the jar B = 1/2, red marble from jar B = 49/99)

Total probability = 74/99 (~3/4)