well, let's recall a whole is always simplified to "1", in this case the rold has been split into 5 equal parts, so each part is 1/5, and the whole is just 1/5 + 1/5 + 1/5 + 1/5 + 1/5 = 5/5 = 1, or just a whole.
well, from the picture, we know that 3/5 in red are 1.5 in length, how much will it be for all 5/5?
[tex]\begin{array}{ccll} a&b\\ \cline{1-2} \frac{3}{5} & 1.5\\[1em] \frac{5}{5}& x \end{array} \implies \cfrac{~~ \frac{3}{5}~~}{\frac{5}{5}}=\cfrac{1.5}{x}\implies \cfrac{~~ \frac{3}{5}~~}{1}=\cfrac{1.5}{x}\implies \cfrac{3}{5}=\cfrac{1.5}{x} \\\\\\ 3x=7.5\implies x=\cfrac{7.5}{3}\implies x=2.5[/tex]
