$$\widetilde{13}=6.6 miles$$ $$\widetilde{23}=4.5 miles$$ $$\widetilde{25}=9.7 miles$$ $$\widetilde{12}=\widetilde{13}-\widetilde{23}=6.6-4.5=2.1 miles$$ $$\widetilde{15}=\widetilde{12}+\widetilde{25}=2.1+9.7=11.8 miles$$