I'm having some trouble setting up an equation for this problem. Any suggestions?

Two children own two-way radios that have a maximum range of 2 miles. One leaves a certain point at 1 p.m. walking due north at a rate of 4 mi/hr. the other leaves the same point at 1:15 p.m., travleing due south 6 mi/hr. When will they be unable to communicate with one another?