You can solve this using a few different concepts. The easy way that may not always work is to notice that 3/6 is equal to 1/2. Next you can quickly determine what 1/2 of 18 is.
The other method is the harder but always successful Brute force way.
Since there are 18 beads total, and you know 3/6 of them are purple, you need to set up your equation as follows:
