, 2016

The shaded and unshaded sections of the ribbon have equal length.

Since there are 5 such sections, then each shaded and unshaded section has length equal to $\frac15$ or $\frac{3}{15}$ of the length of the ribbon.

All measurements which follow are made beginning from the left end of the ribbon.

Point $A$ is located 3 sections from the left end of the ribbon, or at a point $3\times\frac{3}{15}=\frac{9}{15}$ along the length of the ribbon.

Point $D$ is located 4 sections from the left end of the ribbon, or at a point $4\times\frac{3}{15}=\frac{12}{15}$ along the length of the ribbon.

All points are equally spaced, and so points $B$ and $C$ divide the unshaded section between points $A$ and $D$ into 3 equal lengths.

Since $A$ is located $\frac{9}{15}$ of the ribbon length from the left end, and $D$ is located $\frac{12}{15}$ of the ribbon length from the left end, then $B$ is located at $\frac{10}{15}$ of the ribbon length, and $C$ is located at $\frac{11}{15}$ of the ribbon length.

Thus, if Suzy makes a vertical cut at point $C$, the portion of the ribbon to the left of $C$ will be $\frac{11}{15}$ of the size of the original ribbon.

We note that no point is located more than 2 sections from the right end of the ribbon.

That is, no point is located more than $2\times\frac{3}{15}=\frac{6}{15}$ along the length of the ribbon when measured from the right end, and so measurements are taken from the left end of the ribbon.