The answer is 12.5 hours. As shown above, we know the following conditions:

1.？ The distance between a and b is 300 kilometers.

2.？ The speed from b to c is 20km/h.

3.？ With B as the origin, the included angle between B and C is 60.

?

To know whether it will affect city A, we only need to find the distance between A and C (the shortest line segment between two points).

So we can push it like this: (published)

So sin30 = AC/AB = > AC =150km ≤170km, so A city will be affected. Because A, B and C form a right triangle, according to Pythagorean theorem, the distance between B and C is 250Km, so the duration is12.5h. ..

I hope it helps you. Besides, after typing so many words, I drew a picture for you myself. Should we adopt? Haha ~