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