Let "w" represent the number of weeks that have passed.
For Joelle:
The number of rocks in her collection after "w" weeks will be: 18 + 4w
For Lewis:
The number of rocks in his collection after "w" weeks will be: 30 + 3w
To find out after how many weeks they will have the same number of rocks in their collections, we can set up the following equation:
18 + 4w = 30 + 3w
Simplifying the equation:
w = 12
After 12 weeks, Joelle and Lewis will have the same number of rocks in their collections.