Let's call the number of weeks "w".
Joelle's equation: 18 + 4w
Lewis's equation: 30 + 3w
To find the number of weeks when they have the same number of rocks, we need to set their equations equal to each other and solve for "w".
18 + 4w = 30 + 3w
Subtract 3w from both sides:
18 + w = 30
Subtract 18 from both sides:
w = 12
So Joelle and Lewis will have the same number of rocks in their collections after 12 weeks.
To find how many rocks they will each have at that time, we can substitute 12 for "w" in either equation. Let's use Joelle's equation:
18 + 4(12) = 18 + 48 = 66
So they will each have 66 rocks in their collections when the amount of rocks is equal.