According to the observation of meteorological department, there is a typhoon center at B, 240km south of A in a coastal city, and its central wind force is 12.

Solution: 1 According to the meaning of the question, we know that the city will not be affected when it reaches level 4, and the wind force in the typhoon center will remain unchanged. Let's calculate the shortest distance from A to BC, as shown in the figure, that is, AD = AB× SIN30 = 240×1/2 =120km. If it is not affected by the typhoon, the city should be more than 25× from the typhoon center. 120 was affected by typhoon.

2. As shown in the figure, the distance affected by typhoon is EF, where AE = 200km and AF = 200km.

Pythagorean theorem, DE= 160 km, so EF=2DE=320 km, duration =320/20= 16 hours.

3. The maximum wind force is that the typhoon moves towards D, so the maximum wind force is 120/25=4.8.