In order to find the speed at which the boat is traveling, we divide the distance it has traveled by the time elapsed:
2.5/20=0.125 miles/minute
Next, we want to find how long it took for the boat to travel 10 miles. Since it travels 0.125 miles every minute, we just divide:
10/0.125=80
Thus, it took 80 minutes for the boat to reach the bridge