Let the speed of the black cat sheriff be x and the speed of the hamster be X+30.

1/ 15(X+30)= 12

X= 150

Suppose the time from the supermarket to the door of the black cat sheriff is t hours.

150T= 180(T-0.05) (the time for hamsters to arrive at the city gate is T+ 1-4 minutes, which is obtained after unit conversion).

T=0.3

The distance from the supermarket to the city gate is 150 * 0.3 = 45km.