https://gyazo.com/d9053bccd6d4def4ae6538448fd6434d
The probability of (\(4\leq x\leq 8\)) is just the area under the graph between x = 4 and x = 8.
P(\(4\leq x\leq 8\)) = (3+4)*(0.1)/2 + 4*0.05 = 0.35 + 0.2 = 0.55.