The hunt is on: Rich's 13 year old daughter Alexandra is going to give him an introductory trombone concert later in the day, and he wants to purchase earplugs before then. Three stores in the vicinity sell earplugs: EarsPlus, 10 minutes away; ICantBelieveYouBoughtHeraTrombone, 15 minutes away; and KidNoise, 20 minutes away. According to the gravity model, what is the probability that Rich will go to each store?