So Marcy has 12 bracelets and 15 necklaces. It is stated in the problem that this ratio (12:15) is the same as the ratio of Shelly's bracelets to necklaces (x:25). To solve this, we set up a proportion like so:
12/15=x/25 and we can use cross multiplication to solve for x. x=20, so the answer is 20 bracelets.
