We search for red balls (neither yellow or black). There are 6 red balls among all 18 balls. Selecting one ball, there is a chance 6/18 that we pull out a red one. 6/18 is the same as 1/3.
The best way to check theese types of questions is go back in plug in X with your answer so 88+97+80+90(which is x)=360 then divide by 4 and get 90! So yes you are correct good job!
of the shaded portion.