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
The wording of this question is a bit confusing... You can't write a sequence in sigma notation, but rather a series or sum. I think the question is asking you to write the sum of the sequence,

which would be

in sigma notation.
To do this, notice that the denominator in each term is a power of 2, starting with
and ending with
. So in sigma notation, this series is

We are asked to sum the two trinomials

. We can start by using the distributive property to remove the parenthesis, giving us

. Next, when we combine like terms, we get

. Hope this helped!
The third one is the correct choice because the solutions are 2 and -2
The sum of 386+98+1734 is 2218