For A:
You add the probabilities to get the answer to A: = .75, or 3/4
Note that the probability of ALL THREE of them hitting would be 1/3 x 1/4 x 1/5.
For B:
2/3 and 3/4 is the probability of the other two people MISSING (the remainder of 1/3, 1/4)
1.0 = 100% chance to hit the target, so 1.0 x 2/3 x 3/4 = 1/2
Mark's chance of hitting is 1/5, so do 1/2 x 1/5, = 1/10
