What is a part : whole ratio?

It is a ratio that compares a part, to the whole.

Typically ratios compare part to part. For instance if we have 3 Red balls (R) and 2 Green balls (G), then the ratio R:G would be 3:2

However, if we were looking at the red part : whole, it would be 3:5

This is what a fraction shows 3/5, which is the more common way of representing this.