Answer:
About 17 weeks
Explanation:
Given
Ryan:

(weekly)
Sarah:

(weekly)
Required
Determine the number of weeks they have the same cards
Represent the number of weeks with w:
For Both individuals, number of card at any given week is:

So,
For Ryan, the expression is:


For Sarah, the expression is:


To determine the number of weeks, we have to equate both equations:

Collect Like Terms


Solve for w


16.6 implies about 17 weeks, because 16.6 approximates to 17
Conclusively, Ryan and Sarah will have the same number of balls in about 17 weeks