Final answer:
The distance of the lake is calculated by multiplying the speed of the ferry (108 km/h) by the time taken to cross the lake (4 hours), resulting in a distance of 432 kilometers.
Step-by-step explanation:
The distance of the lake can be calculated using the formula for speed, which equals distance divided by time. Given that the fast ferry travels at an average speed of 108 km/h and takes four hours to cross the lake, the distance can be calculated as follows:
Distance = Speed × Time
Distance = 108 km/h × 4 hours
Distance = 432 km
Hence, the lake is 432 kilometers wide.