Let's denote Joelle's number of rocks as J and Lewis's number of rocks as L. We can set up the following system of equations:
J = 18 + 4w (J is the number of rocks Joelle has after w weeks)
L = 30 + 3w (L is the number of rocks Lewis has after w weeks)
To find the week when the number of rocks in their collections is equal, we can set J = L and solve the system of equations:
18 + 4w = 30 + 3w
Now, we can solve for w to find the week when the number of rocks in their collections is equal.