If katy buys 2 dvds every 2 weeks how many will she buy in 12 weeks

To figure this out, we need to set up a proportion.
2:2, the first 2 being how many dvd's she buys, the second being how many weeks.
2:2 = x:12. 
To solve for this, divide how many weeks we will have by how many we do have.
12/2 = 6.
6 is our scale factor, so now we can multiply the number of dvd's by our scale factor.
2 x 6 = 12.
Katy will buy 12 dvd's in 12 weeks.
I hope this helps!