To find the probability of the shaded portion "G", You need to find the total area of the graph.
10 x 10 = 100 units.
G shaded portion = 18 units.
So, 18/100 is the probability of the dart falling on the "G" portion.
You can simplify it to 0.18 which is 18% of chance that it will fall there.
Hope you get it.