Question

After sailing 12 mi, a sailor changed direction and increased the boat's speed by 4 mph. an additional 18 mi was sailed at the increased speed. the total sailing time was 3 h. find the rate of the boat for the first 12 mi.

Answer 1

Let the speed for the first 12 mi be x mi/h, the speed for 18 mi was (x+4) mi/h
thus given that
time=distance/speed 
the average time will be:
3=(12+18)/(x+x+4)
3=30/(2x+4)
solving for x we get
3(2x+4)=30
6x+6=30
6x=24
x=4 mi/hr
Answer: 12 mi/hr