Question 1207221
.
A motorboat travels the distance from one pier to another pier in 4 hours 
and the way back in 5 hours. What is the speed of the boat in still water 
if it travels 70 km with the current in 3.5 hours?
~~~~~~~~~~~~~~~~~~~~


<pre>
70 km with the current in 3.5 hours means the speed downstream is  70/3.5 = 20 km/h.


The trip in 4 hours  downstream with the speed of 20 km/h means that one way distance is 20*4 = 80 km.


Spending 5 hours upstream the distance 80 km means that upstream speed is 80/5 = 16 km/h.


Having the speeds 20 km/h downstream and 16 km/h upstream means that the average of these two values is

    {{{(20+16)/2}}} = {{{36/2}}} = 18 km/h.


This average is the speed of the boat in still water - so, the speed of the motorboat in still water is 18 km/h.    <U>ANSWER</U>
</pre>

Solved.