Two boats started to move towards each other from two docks, located 30 miles apart. One boat moved at a speed of 2.5 miles per hour, the other moved twice as fast. How soon will the boats meet?

Time taken for the boats to meet will be given by:

time = (distance) / (speed)

relative speed = (speed of boat A+speed of boat B)

speed of boat A=2.5 mph

speed of boat B=2 (2.5) = 5 mph

thus

relative speed = (5+2.5) = 7.5 mph

distance=30 miles

thus

time=30/7.5=4 hours

thus the earliest they can meet is 4 hours