Answer
a) $0.83$
b) $0.85$
Work Step by Step
Note that $S =$ {$0, +2, +3, +4$}
a) We want to find $P( charge >0) = P(+2) + P(+3) + P(+4) = 0.35 + 0.33 + 0.15 = 0.83$
b) We want to find $P( charge \leq +3) = P(0) + P(+2) + P(+3) = 0.17 + 0.35 + 0.33 = 0.85$