Samuel has y = 4x + 18 (y represents the number of rocks in his collection, x represents the number of weeks)
Lewis has y = 3x + 30
To find the number of weeks until they have the same number of rocks:
4x + 18 = 3x + 30
x = 12
After 12 weeks, they will have the same number of rocks in their collections.
To find out how many rocks they will each have:
Plug in x = 12 into either equation
Samuel: y = 4(12) + 18 = 66 rocks
Lewis: y = 3(12) + 30 = 66 rocks
They will each have 66 rocks in their collection when the amount is equal.