Please hel[ me with math homework? i really have to get to bed. :(?

monica has a paper bag with 13 red gummy bears, 2 green gummy bears, and 2 yellow gummy bears. When she lets her friend reach into the bag to grab a gummy bear, she wnats the probability of randomly selecting a yellow gummy bear to be 1/4. How many yellow gummy bears should she add to whats already in the bag?