if the ratio of katie's stickers to Jenna's is 7:4 and katie has 21 stickers how many does jenna have?
The long way to answer this would be to set up a proportion and solve for x.
But if you look at the ratio and know Katie has 21 stickers, then Jenna should have 12 because 7 times 3 is 21 and 4 times 3 is 12.