This question rests on the proportion 3/10, and how that might scale up to a larger bag. It says the probability of choosing a strike is 3/10; we can take that to mean that, in a bag of 10 chips, there are 3 strikes. Now, we're dealing with an unknown number of chips, but we are given the number of strikes it contains: 9. What are we multiplying - or scaling - the 3 by to get 9? We'll have to use that same scaling factor to scale up our 10. As an equation, this translates to:
[tex] \frac{3}{10}= \frac{9}{x}[/tex]
Where x is the number of chips in the larger bag.
